COMMUNICATIONS MITTEILUNGEN FROM THE DER KONKOLY OBSERVATORY STERNWARTE OF THE DER UNGARISCHEN AKADEMIE HUNGARIAN ACADEMY OF SCIENCES DER WISSENSCHAFTEN BUDAPEST - SZABADSAGHEGY No. 96. (Vol. 11, Part 3) NORTHERN CEPHEIDS: PERIOD UPDATE AND DUPLICITY EFFECTS L. SZABADOS BUDAPEST, 1991 ISBN 963 8361 34 4 HU ISSN 0238 - 2091 Felelos kiado: Szeidl Bela NORTHERN CEPHEIDS: PERIOD UPDATE AND DUPLICITY EFFECTS ABSTRACT O-C diagrams have been continued for 64 northern Cepheids with the primary aim of studying the effects of duplicity on the pulsation period. Because the light-time effect in the O-C diagrams of binary Cepheids has to be accompanied with properly phased variations in the gamma-velocity, the radial velocity of the programme stars has been studied, as well. Light-time effect is suspected in the O-C diagram of FM Aql, RW Cam, Y Lac, and RS Ori, and confirmed for AW Per. One or more phase jumps are present or suspected in the O-C diagram of 19 northern Cepheids (FF Aql, BY Cas, DD Cas, DL Cas, X Cyg, SU Cyg, SZ Cyg, DT Cyg, V532 Cyg, V924 Cyg, TX Del, DX Cem, X Lac, CV Mon, RS Ori, SV Per, SZ Tau, T Vul, X Vul). In addition to the Cepheids with known spectroscopic orbit, the spectroscopic binary nature based on the variability of the gamma-velocity has been confirmed, revealed or suspected for the majority of the programme stars. The most probable new spectroscopic binary Cepheids are: KL Aql, Eta Aql, SU Cas, V636 Cas, BZ Cyg, MW Cyg, V386 Cyg, W Gem, RZ Gem, AD Gem, RS Ori, SV Per, SW Tau, T Vul, and U Vul. A preliminary value of the orbital period is suggested for Eta Aql, SU Cas, RZ Gem, T Vul, and U Vul. INTRODUCTION Period changes of more than a hundred northern Cepheids were studied in a series of papers (Szabados, 1977, 1980 and 1981, hereinafter referred to as Papers I, II and III, respectively). As a result, the observed period changes were compared with the theoretical ones, predicted by the stellar evolutionary calculations (Szabados, 1983). In addition to the evolutionary changes (manifested in parabolic O-C graphs), two special kinds of period variations were also revealed in several cases, both of them being characteristic of binary Cepheids: 1. light-time effect due to the orbital motion, 2. phase jump, i.e. a stepwise O-C graph. The origin of this latter type of the period change has not been clarified yet, but the phase jumps always occur in Cepheids having a companion star. Later on, it became obvious that the extension of that study to southern Cepheids was of importance because the period variation of most Cepheids with negative declination had not been followed closely. The investigation of 44 bright southern Cepheid variables was published recently (Szabados, 1989 = Paper IV). Because the programme stars were selected arbitrarily, the primary goal of Paper IV was the study of duplicity effects in the O-C diagram, and no special attention was paid to follow the evolutionary period changes. A companion star can also alter the observable gamma-velocity of the given variable, if the orbital inclination significantly differs from zero. A light-time effect in the O-C diagram has to be accompanied with properly phased gamma-velocity variations of the same period, and the amplitude of the oscillation in the O-C diagram is not independent from that deduced from the diagram gamma-velocity vs. time. For this reason the study of period changes was supplemented with a comprehensive study of gamma-velocity variations of the programme stars. By the end of the eighties it became obvious that the frequency of the Cepheid binaries is much higher than thought before (Szabados, 1990b). The increase of the known spectroscopic binaries among Cepheids is mostly a result of the ultraviolet spectroscopy made with the IUE-satellite, and the thorough radial velocity studies performed in the last decade. In some cases, however, the available, sometimes sporadic, radial velocity measurements were even sufficient for revealing the orbital effect, or determining the orbital period (see e.g. Szabados, 1990a). The aim of the present paper is to analyse the period changes of the known binary Cepheids of the northern sky, in order to study the effects of duplicity, and to search for gamma-velocity variations in the case of suspected binary Cepheids with a declination larger than zero. Therefore the sample of stars studied here is selected arbitrarily: it contains 64 stars of various brightness (including four Population II Cepheids). Even some of the brightest Cepheids have been omitted (e.g. delta Cephei itself), being very probably single stars. Their period changes have to be also studied, but this paper is the last one in the extensive series on this topic, and the similar studies in the future will concentrate on individual Cepheids. Unfortunately the variable star astronomers have lost their interest in the regular photometry of the Cepheid variables, and it is to be afraid that the subtle but important period changes in these stars will pass unnoticed. The programme stars are arranged in alphabetical order of constellations. The list of the Cepheids involved in this study is as follows (the ordinal number following the name of the Cepheid gives the page number where the discussion on the given star begins): Cepheid page Cepheid page Cepheid page SZ Aql 129 SZ Cyg 161 BG Lac 197 TT Aql 131 TX Cyg 163 T Mon 198 FF Aql 132 VZ Cyg 164 SV Mon 200 FM Aql 134 BZ Cyg 166 CV Mon 202 KL Aql 135 DT Cyg 167 V465 Mon 203 V572 Aql 136 MW Cyg 169 RS Ori 204 V1344 Aql 137 V386 Cyg 170 GQ Ori 206 Eta Aql 138 V532 Cyg 172 SV Per 206 RT Aur 140 V924 Cyg 174 VX Per 208 AN Aur 142 V1334 Cyg 175 AS Per 209 RW Cam 143 V1726 Cyg 177 AW Per 210 SU Cas 145 TX Del 177 V440 Per 212 SZ Cas 147 W Gem 178 S Sge 213 RY Cas 148 RZ Gem 180 SW Tau 216 DD Cas 150 AD Gem 182 SZ Tau 217 DL Cas 151 DX Gem 183 S Vul 219 IX Cas 153 zeta Gem 185 T Vul 220 V636 Cas 154 V Lac 189 U Vul 223 IR Cep 155 X Lac 191 X Vul 224 V351 Cep 156 Y Lac 193 SV Vul 226 X Cyg 157 Z Lac 194 SU Cyg 159 RR Lac 196 In addition to the binary Cepheids, some other Cepheid variables not having a companion were also studied, provided that the construction of a new O-C diagram contains relevant new information as compared with the original O-C plot published in Papers I-III. The new piece of information can be a recent period change, or larger accuracy due the new photoelectric O-C residuals overwhelming in the present O-C diagrams. Similarly, the shape of the O-C diagram is differently interpreted in the case of several binary Cepheids, as compared with the previous one. Besides the reasons listed above, the main cause of the modified shape of the O-C graphs is a recently discovered phase jump. Since the duplicity effects in the O-C diagram (light-time effect and phase jump) are usually very subtle, only the photoelectric observations have been taken into account whenever possible. In a number of cases, however, photographic observations were also used when constructing the new O-C diagram, and for seven Cepheids (SU Cyg, VZ Cyg, W Gem, RZ Gem, zeta Gem, X Lac, SV Per) the early visual observations were also analysed. These latter exceptional cases are examples for either a parabolic O-C graph or an early phase jump, therefore the visual observations even from the last century are of primary importance. The O-C residuals taken from Papers I-III have the same weight as that assigned to them originally. As far as the visual and the photographic observations are concerned, the O-C residuals based on such observations with a weight less than unity have not been used here. The recently published photoelectric light curves are often superior to the previous ones. The new normal light curve determined for more than twenty programme stars showed a marked difference as compared with the previously used normal curve. In these cases the O-C residuals taken from Papers I-III were corrected accordingly. It has to be noted that, although solely photoelectric data have been used from among the recently published observations, several photoelectric observational series have been omitted, i.e. those obtained in the IR-region (e.g. Schmidt, 1976; Welch et al., 1984) because of the uncertain phase shift between the moments of maxima in the blue and infrared bands. In the following discussion there are usually two tables and one figure for each Cepheid. The successive columns of the tables containing information on the O-C residuals give the following data: 1. Moment of normal maximum (an asterisk indicates that the given moment is a new one, not appearing in Papers I-III), 2. The corresponding epoch, 3. O-C residual (in days), 4. The weight assigned to the residual (a blank character means that the given residual has not been used in the curve-fitting procedure), 5. Source of the observational data. The epoch and the O-C residual have been obtained using the linear ephemeris given in the discussion on each Cepheid. The determination of the gamma-velocities was performed in a similar manner as described in Paper IV. The successive columns in the Tables of the gamma-velocities contain the following data: 1.-2. Mean date of the observations and its formal standard deviation, 3.-4. gamma-velocity and its formal standard deviation, 5. Number of radial velocity observations used, 6. Source of the observational data. Note that the uncertainty in the zero-point of the individual radial velocity measurement series tends to increase the standard deviation given in the tables, but no allowance was made for the zero-point differences. The gamma-velocity of a programme star is considered to be variable if its variation is larger than five km/s. The uncertainty of the zero-point hardly exceeds one km/s in the case of the modern radial velocity observations. The gamma-velocity of the Cepheid-binaries with known orbit is not analysed here. The figures visualizing the tabular data are usually divided into two parts. The upper panel shows the O-C diagram of the given variable. Filled circles denote the O-C residuals based on photoelectric observations, while open circles are those of photographic (or visual, in the case of the seven Cepheids listed above) origin. The size of the circles refers to the weight assigned to the O-C residual. The least squares fit (usually linear or parabolic) is also shown. It has to be noted, however, that the O-C residuals earlier than J.D. 2420000 are not plotted, even if they are listed in the corresponding table because those particular residuals were used during the curve-fitting procedure. In several cases sections of the O-C graph not studied here are also drawn for convenience (without listing the corresponding visual or low quality photographic O-C residuals). The ephemeris used for obtaining the O-C residuals is indicated at the top of the figure. The lower panel of the figures shows the individual gamma-velocities as a function of the Julian Date. The reliability of the data points can be estimated from the error bars (if the standard deviation exceeds the size of the dot). Again, the figure does not show the tabular values before J.D. 2420000. REMARKS ON THE INDIVIDUAL VARIABLES SZ Aquilae The three more recent photoelectric observational series confirm the previous conclusion (Paper III) about the continuous period increase. The new O-C diagram has been constructed using the elements: C = 2443807.165 + 17.140554d*E (1) +-.032 +-.000233 The momentary value of the period is as follows: P = 17.140554d + 3.29d*E*10^-6 (2) +-.000233 +-.64 Table 1. O-C residuals for SZ Aql Norm.max. E O-C W Reference JD2400000+ 29513.184 -834 1.487 Ahnert (1951) 29838.681 -815 1.308 Erleksova (1960) 31638.130 -710 0.758 Erleksova (1960) 32460.734 -662 0.616 Erleksova (1960) 33112.198 -624 0.739 3 Eggen (1951) 33454.820 -604 0.550 Erleksova (1960) 34414.613 -548 0.472 Erleksova (1960) 35494.235 -485 0.239 Erleksova (1960) 35580.096 -480 0.397 1 Walraven et al. (1958) 36231.149 -442 0.109 Erleksova (1960) 37156.786 -388 0.156 2 Mitchell et al. (1964) 37945.611 -342 0.515 1 Williams (1966) 40910.526 -169 0.115 3 Pel (1976) 41338.940* -144 0.015 3 Feltz & McNamara (1980) 42898.836 -53 0.120 2 Dean (1977) 43807.095 0 -0.070 3 Szabados (1981) 44441.228* 37 -0.137 2 Eggen (1983b) 44612.842* 47 0.071 3 Moffett & Barnes (1984) Figure 1. O-C diagram of SZ Aql The O-C residuals are listed in Table 1 and shown plotted in Figure 1. The small number of radial velocity data (Joy, 1937; Barnes et al., 1988) does not allow the determination of accurate gamma-velocities. TT Aquilae This bright Cepheid was frequently observed photometrically in the last two decades, thus making possible the reliable period determination based on photoelectric observations alone (see Table 2 and Figure 2). The current ephemeris is as follows: C = 2443810.958 + 13.754954d*E (3) +-.014 +-.000043 Although the O-C residuals based on the photographic and the early photoelectric observations suggest a wave-like pattern, this tendency disappears after J.D. 2440000, as if a sudden change in the pulsation period occurred. It is worth mentioning that the Julian Dates in Connolly et al.'s (1982) paper need a correction of -1 day. The revised pulsation period in their paper is also an artifact of this mistake. The gamma-velocities of TT Aql are collected in Table 3, and are shown plotted in the lower panel of Figure 2. A slight variation in the average radial velocity cannot be excluded but further high quality observations Table 2. O-C residuals for TT Aql Norm.max. E O-C W Reference JD2400000+ 29385.963 -1016 0.038 Erleksova (1960) 30564.956 -963 0.019 Erleksova (1960) 31527.765 -893 -0.019 Erleksova (1960) 32449.337 -826 -0.029 Erleksova (1960) 33109.654 -778 0.050 3 Eggen (1951) 33494.741 -750 -0.002 Erleksova (1960) 34416.368 -683 0.044 Erleksova (1960) 35282.936 -620 0.049 2 Irwin (1961) 35502.954 -604 -0.012 Erleksova (1960) 35558.014 -600 0.028 2 Walraven et al. (1958) 36218.152 -552 -0.071 Erleksova (1960) 37208.525 -480 -0.055 3 Mitchell et al. (1964) 37937.601 -427 0.008 1 Williams (1966) 40413.494* -247 0.010 2 Feltz & McNamara (1980) 40867.193 -214 -0.205 1 Evans (1976) 40867.331 -214 -0.067 3 Pel (1976) 41266.313* -185 0.021 2 Feltz & McNamara (1980) 41912.777 -138 0.003 3 Landis (1976) 42916.670 -65 -0.216 2 Dean (1977) 43343.343* -34 0.053 3 Moffett & Barnes (1984) 43810.884 0 -0.074 3 Szabados (1981) 43865.980* 4 0.002 3 Moffett & Barnes (1984) 44443.719* 46 0.033 3 Connolly et al. (1982) 44443.719* 46 0.033 3 Coulson et al. (1985) 44512.589* 51 0.128 3 Eggen (1983b) 44691.332* 64 0.057 2 Connolly et al. (1982) 44828.824* 74 -0.001 2 Coulson et al. (1985) Figure 2. Upper panel: O-C diagram of TT Aql Lower panel: gamma-velocities for the same Cepheid Table 3. gamma-velocities of TT Aql J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 22002 152 -4.1 3.2 3 Joy (1937) 25809 260 -0.4 1.8 7 Joy (1937) 41031 146 4.0 0.2 7 Evans & Lyons (1986) 43317 31 3.0 0.4 3 Evans & Lyons (1986) 43369 36 0.8 1.2 12 Wilson et al. (1989) 43688 75 5.5 1.0 16 Barnes et al. (1987) 44036 47 2.1 1.8 6 Barnes et al. (1987) 44180 3 0.1 1.4 4 Coulson et al. (1985) 44427 10 1.3 0.5 27 Coulson et al. (1985) 44778 1 2.2 2.5 2 Coulson et al. (1985) 45133 521 3.3 0.2 7 Evans & Lyons (1986) 47745 34 3.9 0.4 15 Samus (1990) 48044 42 3.8 0.4 18 Samus (1990) are necessary to make a firm statement. Leonard and Turner (1986) summarized the available information on duplicity of TT Aql. Although the photometric test by Madore and Fernie (1980) suggests the presence of a bright blue companion, no other positive evidence is available (see Evans, 1985; Coulson et al., 1986). FF Aquilae This variable is one of the most popular Cepheids among the observers, therefore a number of new results has been achieved. As to its photometry, the new O-C diagram (see Figure 3 and Table 4) clearly shows the existence Figure 3. O-C diagram of FF Aql Table 4. O-C residuals for FF Aql Norm.max. E O-C W Reference JD2400000+ 25096.650 -3686 0.171 3 Huffer (1931) 25490.067 -3598 0.146 3 Huffer (1931) 25811.968 -3526 0.139 3 Huffer (1931) 32960.946 -1927 0.091 3 Eggen (1951) 34628.641 -1554 0.127 3 Szabados (1977) 35625.598 -1331 0.065 3 Walraven et al. (1958) 36099.567 -1225 0.115 2 Svolopoulos (1960) 37320.127 -952 0.109 3 Mitchell et al. (1964) 39019.100 -572 0.126 3 Wisniewski & Johnson (1968) 40266.395* -293 0.030 1 Feltz & McNamara (1980) 40789.448* -176 -0.016 1 Feltz & McNamara (1980) 40811.901 -171 0.082 3 Pel (1976) 41245.539* -74 0.039 2 Feltz & McNamara (1980) 41576.448 0 0.099 3 Szabados (1977) 43342.376* 395 0.007 3 Moffett & Barnes (1984) 43369.211* 401 0.017 3 present paper 43615.129* 456 0.033 1 Henden (1979) 43673.206* 469 -0.012 3 Moffett & Barnes (1984) 43731.324* 482 -0.016 3 present paper 44035.355* 550 -0.009 2 Moffett & Barnes (1984) 44853.537* 733 -0.008 3 Arellano Ferro (1984) 46284.270* 1053 0.025 1 "Carlsberg" (1989) 46624.056* 1129 0.020 2 present paper 47455.601* 1315 -0.029 1 Usenko (1990a) of a phase jump, a phenomenon that has already been suspected by Evans et al. (1990b) on the basis of the new radial velocity data. The O-C residuals in Table 4 have been computed using the ephemeris: C = 2441576.349 + 4.470936d*E (4) +-.009 +-.000014 and this ephemeris is valid for predicting the maxima after J.D. 2443000. Between J.D. 2433000 and 2442000 the following formula gives the best fit to the O-C residuals: C = 2441576.423 + 4.470918d*E (5) +-.011 +-.000010 Therefore the pulsation period remained constant during the two sections of the O-C graph, while the amount of the phase shift is about 0.08 day (or 0.02 phase). The phase jump occurred between J.D. 2442000 and 2443000. The different values of the pulsation period as determined from the O-C diagrams for the maximum and median brightness (see Paper I, page 92) can now be interpreted as a minor change in the light curve shape similarly to the other known case of SU Cyg (see Paper I and this paper, p. 159). The radial velocity measurements of FF Aql are not analysed here because the study of Evans et al. (1990b) is so thorough and complete. Their paper includes a new determination of the orbit, and also contains all the available information concerning the companions to FF Aql. The only contribution here to the spectroscopic study is a single radial velocity measurement listed in Table 109. FM Aquilae The new O-C diagram based on only photoelectric observations (see Table 5 and Figure 4) gives a slightly longer period than that determined in Paper II. The new ephemeris is as follows: C = 2442678.253 + 6.114265d*E (6) +-.006 +-.000008 Figure 4. Upper panel: O-C diagram of FM Aql Lower panel: gamma-velocities for the same Cepheid Table 5. O-C residuals for FM Aql Norm.max. E O-C W Reference JD2400000+ 32962.724 -1589 0.038 3 Eggen (1951) 35292.245 -1208 0.024 2 Irwin (1961) 35500.086 -1174 -0.020 2 Walraven et al. (1958) 37187.583 -898 -0.060 3 Mitchell et al. (1964) 40819.509 -304 -0.007 3 Pel (1976) 41223.078* -238 0.020 3 Feltz & McNamara (1980) 42678.229 0 -0.024 3 Szabados (1980) 44335.221* 271 0.002 3 Moffett & Barnes (1984) 44830.486* 352 0.012 3 Eggen (1985) 44983.348* 377 0.017 3 Moffett & Barnes (1984) Table 6. gamma-velocities of FM Aql J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 27219 278 -14.5 1.7 8 Joy (1937) 44391 192 -10.8 1.4 9 Barnes et al. (1988) 44821 45 -12.8 2.3 4 Barnes et al. (1988) 47648 4 -12.4 0.2 6 Samus (1990) Moreover, an apparent period variation caused by the light-time effect may be superimposed on the O-C graph (see Figure 4). The estimated period (about 2800 days) and the amplitude of the sinusoidal variation implies an orbital radial velocity variation that might be easily detected. The available radial velocity measurements (see Table 6), however, do not support the variable gamma-velocity hypothesis. Nevertheless, there have been evidence in favour of a blue companion. Madore (1977) derived a B9V type photometric companion, while Pel (1978) concluded that FM Aql had a peculiar colour - colour loop. The ultraviolet spectrum of this Cepheid, however, does not indicate the presence of a companion earlier than A0V (Evans et al. 1990a). Further spectroscopic observations are desirable to settle this problem. KL Aquilae There are no newly published photometric observations on this neglected Cepheid, thus the previous O-C diagram (Paper II, p. 53) cannot be replaced with a recent one. The existing radial velocity measurements, however, have not been analysed before. As one can see in Table 7, the gamma-velocities show a strong variation on a time-scale of several hundred days. KL Aql seems to be a new spectroscopic binary Cepheid, worthy of immediate observation. Table 7. gamma-velocities of KL Aql J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 27543 124 -1.5 2.0 6 Joy (1937) 28097 1 -6.8 4.5 1 Joy (1937) 28396 21 -45.5 1.4 3 Abt (1973) 44735 129 -0.5 1.2 12 Harris & Wallerstein (1984) 45167 48 1.0 2.0 5 Harris & Wallerstein (1984) V572 Aquilae All the previous photoelectric observations have been analysed again, because Henden's (1979) data form a better normal light curve than that used in Paper I. Therefore the moments of the normal maxima listed in Table 8 are different from the corresponding values listed in Paper I (p. 70). In spite of the reliable new normal light curve, the O-C residuals widely scatter around the best fitting line (see Figure 5) described with the formula: C = 2441921.693 + 3.768001d*E (7) +-.088 +-.000080 Figure 5. O-C diagram of V572 Aql Table 8. O-C residuals for V572 Aql Norm.max. E O-C W Reference JD2400000+ 35666.826 -1660 0.015 3 Walraven et al. (1958) 36789.753 -1362 0.077 3 Oosterhoff (1960) 41921.334 0 -0.359 3 Szabados (1977) 43734.368* 481 0.267 3 Henden (1979) These deviations are possibly caused by period changes (a previous change in the pulsation period was suspected in Paper I). Therefore the elements given here and in Table 110 are only tentative. No radial velocity measurements have been published about this star. V1344 Aquilae Arellano Ferro's (1984) recent photoelectric observations form a new normal light curve superior to the previous one (Kovacs and Szabados, 1979), therefore these earlier published observations were re-analysed when constructing the O-C diagram. The O-C residuals plotted in Figure 6 and listed in Table 9 have been calculated with the ephemeris: C = 2443398.184 + 7.476787d*E (8) +-.015 +-.000104 Figure 6. Upper panel: O-C diagram of V1344 Aql Lower panel: gamma-velocities for the same Cepheid Table 9. O-C residuals for V1344 Aql Norm.max. E O-C W Reference JD2400000+ 43016.896 -51 0.028 3 Kovacs & Szabados (1979) 43958.889 75 -0.054 3 Kovacs & Szabados (1979) 44482.301* 145 -0.017 3 Fernie & Garrison (1981) 44781.430* 185 0.040 3 Arellano Ferro (1984) 44788.871* 186 0.005 3 Eggen (1985) Table 10. gamma-velocities of V1344 Aql J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 44424 27 -2.3 0.1 16 Balona (1981) 44528 15 -2.7 0.2 8 Balona (1981) 44832 6 0.2 0.6 8 Arellano Ferro (1984) The available radial velocity data, although being very accurate, are not sufficient to draw a firm conclusion on the variability of the gamma-velocity (see Table 10). If V1344 Aql is really a spectroscopic binary, then the orbital period has to be relatively short (several hundred days). An extension of the observations to a longer time-base both in photometry and spectroscopy would be necessary. Eta Aquilae Because the photoelectric observations obtained by Moffett and Barnes (1984) offered a better normal ligth curve than that used previously, this new normal curve has been used for determining the moments of normal maxima for the photoelectric observations published in the eighties. In order to eliminate the systematic difference in the phase of the maximum light between the recent and the previous normal curve, a correction of -0.029 day has been applied to the photoelectric O-C residuals published in Paper II. Both the corrected and the recently determined O-C residuals are listed in Table 11. A parabolic fit, i.e. a continuously increasing pulsation period is still the most appropriate interpretation of the O-C graph (see Figure 7). The O-C residuals have been calculated using the Figure 7. Upper panel: O-C diagram of Eta Aql Lower panel: gamma-velocities for the same Cepheid ephemeris: C = 2442794.726+ 7.176726d*E (9) +-.006 +-.000014 The instantaneous value of the period can be predicted as follows: p = 7.176726d+ 3.16d*E*10^-8 (10) +-.000014 +-1.13 There is no significant difference between this value and that determined in Paper II. Table 11. O-C residuals for n Aql Norm.max. E O-C W Reference JD2400000+ 22585.188 -2816 0.122 3 Wylie (1922) 23991.859 -2620 0.155 1 Pettit & Nicholson (1933) 26144.796 -2320 0.074 2 Bernheimer (1931) 33041.532 -1359 -0.023 3 Eggen (1951) 33070.289 -1355 0.027 3 Stebbins et al. (1952) 34613.324 -1140 0.066 3 Szabados (1980) 35295.055 -1045 0.008 2 Irwin (1961) 35574.965 -1006 0.025 2 Walraven et al. (1958) 36141.892 -927 -0.009 1 Oke (1961) 37283.027 -768 0.027 3 Mitchell et al. (1964) 37857.144 -688 0.005 1 Williams (1966) 38926.492 -539 0.021 3 Wisniewski & Johnson (1968) 39751.864 -424 0.070 2 Sudzius (1969) 39888.119 -405 -0.033 2 Schmidt (1971) 40239.753* -356 -0.058 3 Feltz & McNamara (1980) 40857.010* -270 0.000 2 Feltz & McNamara (1980) 40928.825 -260 0.048 3 Pel (1976) 40957.453 -256 -0.031 2 Evans (1976) 42127.285* -93 -0.005 3 Depenchuk (1980) 42794.752 0 0.026 3 Szabados (1980) 42945.483 21 0.046 2 Dean (1977) 43203.745* 57 -0.054 2 Depenchuk (1980) 43311.439* 72 -0.011 2 Dean (1981) 43340.153* 76 -0.004 3 Moffett & Barnes (1984) 43864.047* 149 -0.011 3 Moffett & Barnes (1984) 44373.617* 220 0.011 2 Schmidt & Parsons (1982) The study of the radial velocity observations, however, gives more novelty. It is a well-known fact that Eta Aql belongs to a binary system (Mariska et al., 1980). Based on the IUE spectra, Bohm-Vitense and Proffitt (1985) derived an A1V companion of Delta V = 4.6 mag. Jacobsen and Wallerstein (1981) suspected long period changes in the systemic radial velocity. As a matter of fact, the analysis of the radial velocity data collected from the literature (see Table 12 and Figure 7) strengthens their conclusion on the variability of the gamma-velocity. It is not clear, however, what period can be assigned to the gamma-velocity changes. The formal period search resulted in a value as short as 926 days. The deviations of the O-C residuals from the fitted parabola clearly show a Table 12. gamma-velocities of Eta Aql J.D. sigma v gamma sigma n Reference 2400000+[d] [km/s] [km/s] 14129 10 -16.7 0.9 13 Belopolski (1897) 14517 32 -15.0 0.6 28 Wright (1899) 19277 366 -13.7 2.1 3 Spencer Jones (1928) 21429 233 -19.6 1.2 4 Abt (1973) 23653 43 -14.4 0.6 28 Jacobsen (1926) 24226 173 -13.4 0.4 57 Henroteau (1928) 24869 461 -17.6 0.9 6 Abt (1973) 25084 23 -18.9 0.8 17 Henroteau & Vibert (1929) 29873 12 -14.6 1.2 4 Jacobsen (1961) 33292 190 -21.6 0.4 22 Jacobsen (1954) 34258 15 -19.8 2.0 2 Jacobsen (1954) 34548 32 -15.3 0.6 14 Jacobsen (1.961) 40502 139 -13.3 1.0 5 Lloyd Evans (1980) 43049 33 -14.3 0.7 3 Jacobsen & Wallerstein (1981) 43141 206 -16.3 0.3 7 Beavers & Eitter (1986) 43384 4 -17.9 1.3 11 Wilson et al. (1989) 43828 206 -15.2 0.7 29 Barnes et al. (1987) 44290 143 -15.4 0.2 18 Jacobsen & Wallerstein (1981) 46033 157 -16.0 0.2 18 Jacobsen & Wallerstein (1987) 47027 1 -12.5 0.7 1 Samus (1990) sinusoidal pattern at this period, as if it were a light-time effect, but this is too subtle to detect with an eye inspection in Figure 7. Further extensive radial velocity measurements are necessary to find the correct value of the spectroscopic orbital period. RT Aurigae Table 13. O-C residuals for RT Aur Norm.max. E O-C W Reference JD2400000+ 29603.272 -3251 -0.035 3 Bennett (1941) 33141.392 -2302 0.025 3 Eggen et al. (1957) 35799.601 -1589 0.029 3 Prokof'yeva (1961) 35881.611* -1567 0.018 3 Bahner & Mavridis (1977) 36202.239* -1481 0.021 3 Bahner & Mavridis (1977) 36616.072* -1370 0.024 2 Bahner & Mavridis (1977) 37339.350 -1176 0.032 3 Mitchell et al. (1964) 37995.423 -1000 -0.058 2 Williams (1966) 38920.047 -752 -0.027 3 Wisniewski & Johnson (1968) 39359.960 -634 -0.041 3 Takase (1969) 40843.831* -236 0.007 2 Feltz & McNamara (1980) 40996.642* -195 -0.038 2 Evans (1976) 41429.115 -79 -0.036 3 Winzer (1973) 41723.711 0 0.032 3 Szabados (1977) 43539.286* 487 -0.025 2 Moffett & Barnes (1984) 44106.001* 639 0.003 3 Moffett & Barnes (1984) 44534.795* 754 0.055 2 Eggen (1985) The eight new O-C residuals supplemented with the earlier photoelectric O-C values (see Table 13) confirm the value of the pulsation period as determined in Paper I. The O-C diagram has been calculated using the formula: C = 2441723.679 + 3.728198d*E (11) +-.006 +-.000005 Although a very long (10000 - 15000 days) wave may be superimposed on the straight line in Figure 8, no spectroscopic confirmation of the light-time effect can be deduced from the available radial velocity data (Table 14). Leonard and Turner (1986) summarized the various arguments for and against duplicity of RT Aur and concluded that this Cepheid probably does not have a bright blue companion. Figure 8. Upper panel: O-C diagram of RT Aur Lower panel: gamma-velocities for the same Cepheid Table 14. gamma-velocities of RT Aur J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 18230 18 21.5 0.6 24 Petrie (1932) 21210 65 19.5 0.6 30 Kiess (1917) 23723 30 20.6 1.3 6 Petrie (1932) 24955 253 20.1 0.7 19 Petrie (1932) 40979 9 21.0 0.3 4 Evans (1976) 43449 59 20.0 1.4 9 Wilson et al. (1989) 43457 275 18.7 0.4 5 Beavers & Eitter (1986) 43963 245 18.0 0.8 25 Barnes et al. (1987) 45717 9 20.4 0.1 45 Gieren (1985) 46866 1 20.4 0.5 2 Samus (1990) AN Aurigae Berdnikov's (1987) recent photometry confirms the period change suspected in Paper III but the phase jump interpretation does not seem to be correct. The O-C residuals listed in Table 15 have been obtained by the formula: C = 2443799.022 + 10.289563d*E (12) +-.017 +-.000036 The photoelectric O-C residuals are plotted in Figure 9. It should be noted that Berdnikov's (1987) photoelectric observations do not support the change in the light curve shape suspected in Paper III. Nevertheless, AN Aur is a binary Cepheid, since the radial velocity observations show a variation in the gamma-velocity. In Figure 10 the open circles denote Joy's (1937) radial velocity data, while Samus' (1990) observations are plotted as filled circles. Zero phase is chosen arbitrarily, the pulsation period is according to Eq.(12). The deviation in the average radial velocity is even more obvious if the phase shift due to the period change occurred after the epoch of Joy's observations is also taken into account. Madore (1977) estimates a B5 photometric companion. Further spectroscopic observations of this Cepheid would be of primary importance. Figure 9. O-C diagram of AN Aur Table 15. O-C residuals for AN Aur Norm.max. E O-C W Reference JD2400000+ 36832.907 -677 -0.081 2 Oosterhoff (1960) 36833.041 -677 0.053 3 Weaver et al. (1960) 43799.026 0 0.004 3 Szabados (1981) 46299.383* 243 -0.003 3 Berdnikov (1987) Figure 10. Radial velocity observations of AN Aur folded with the period 10.289563 days. Zero phase is chosen arbitrarily. Open circles: Joy's (1937) data, filled circles: Samus'(1990) observations RW Camelopardalis The O-C diagram of this binary Cepheid supplemented with the recent O-C residuals (see Table 16) is plotted in Figure 11. The O-C residuals have been calculated using the ephemeris: C = 2443840.694 + 16.415015d*E (13) +-.020 +-.000067 If the wave-like pattern is interpreted in terms of the light-time effect, an orbital period of about 7000 days characterizes the system. In Table 16. O-C residuals for RW Cam Norm.max. E O-C W Reference JD2400000+ 36174.873 -467 -0.009 3 Bahner & Mavridis (1977) 36831.480 -427 -0.003 3 Oosterhoff (1960) 36831.546 -427 0.063 3 Weaver et al. (1960) 36880.608 -424 -0.120 3 Bahner et al. (1962) 39113.340 -288 0.170 3 Wamsteker (1972) 39786.182 -247 -0.003 2 Szabados (1981) 43840.515 0 -0.179 3 Szabados (1981) 44382.394* 33 0.005 2 Moffett & Barnes (1984) 45038.953* 73 -0.037 3 Moffett & Barnes (1984) 45695.611* 113 0.020 3 Berdnikov (1986) 46483.539* 161 0.028 2 present paper 47304.378* 211 0.116 2 present paper Figure 11. Upper panel: O-C diagram of RW Cam Lower panel: gamma-velocities for the same Cepheid Table 17. gamma-velocities of RW Cam J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 22740 1 -26.1 4.5 1 Joy (1937) 25050 340 -22.1 2.3 5 Joy (1937) 26364 288 -26.1 2.0 6 Joy (1937) 44456 160 -27.1 1.0 18 Barnes et al. (1988) 45168 223 -24.8 1.2 12 Barnes et al. (1988) Paper III a somewhat shorter orbital period was suggested but Joy's (1937) radial velocity measurements (see Table 17) prefer this longer value. The amplitude of the sinusoidal O-C variation is rather large, giving rise to considerable gamma-velocity changes. Therefore the variation in the gamma-velocity has to be larger than it has been observed till now. The phasing of the radial velocity data with the suspected orbital period, however, shows that RW Cam has never been observed spectroscopically during the phases when the Cepheid is approaching the observer. According to Figure 11, this orbital phase occurs just in the nineties, so any radial velocity study to be performed in the near future would answer the question whether the light-time effect interpretation is correct. If this is not the case, the other plausible interpretation of the O-C graph would be the occurrence of a phase jump. In any case, a regular coverage of the light variation is also desirable. In addition to the previously published photometric evidence, the blue companion of RW Cam has been pointed out in the IUE spectra (Bohm-Vitense and Proffitt, 1985). SU Cassiopeiae The O-C diagram of this bright Cepheid based on only photoelectric observations is shown in Figure 12 (see also Table 18). The pulsation period has been constant since the discovery of the light variation of SU Cas. The current ephemeris C = 2441645.913 + 1.949325d*E (14) +-.003 +-.000003 is practically the same as derived in Paper I. The same conclusion has been drawn by Rhode (1990a). On the contrary, the study of the available radial velocity observations gives more interesting results. SU Cas also belongs to a binary system (Evans, 1985). This finding has been confirmed photometrically (Usenko, 1990b): the position of SU Cas on the two-colour diagram can be explained by assuming an A0 companion. In the light of these facts it is worth looking for any change in the gamma-velocity of this Cepheid (see Table 19). There are four possible values of the orbital period: 462.5, 928, 1375 and 1682 days. Although any data set can be folded with a "best fitting" sinusoid, and the periodicity does not necessarily bear physical significance, the 462.5 day period seems to be not simply an artifact of the data distribution. Of course, the "orbital velocity curve" plotted in Figure 13 has to be confirmed by additional radial velocity measurements. Table 18. O-C residuals for SU Cas Norm.max. E O-C W Reference JD2400000+ 30404.167 -5767 0.011 3 Walter (1943y) 30905.119 -5510 -0.013 3 Groeneveld (1944) 35755.041 -3022 -0.012 3 Prokof'yeva (1961) 36121.522* -2834 -0.004 3 Bahner & Mavridis (1977) 36199.516 -2794 0.017 2 Svolopoulos (1960) 36836.942 -2467 0.014 2 Bahner et al. (1962) 37439.297 -2158 0.027 3 Mitchell et al. (1964) 38384.671 -1673 -0.021 3 Wisniewski & Johnson (1968) 39055.269 -1329 0.009 3 Milone (1970) 39361.299 -1172 -0.005 3 Takase (1969) 39447.074 -1128 0.000 3 Wamsteker (1972) 39751.198 -972 0.029 2 Sudzius (1969) 39864.198 -914 -0.032 3 Reed (1968) 40180.041* -752 0.020 2 Feltz & McNamara (1980) 40963.647* -350 -0.002 3 Feltz & McNamara (1980) 41645.925 0 0.012 3 Szabados (1977) 41930.480 146 -0.034 3 Gieren (1976) 43347.688* 873 0.014 3 Niva & Schmidt (1979) 43690.749* 1049 -0.006 3 Moffett & Barnes (1984) 44178.083* 1299 -0.003 3 Moffett & Barnes (1984) 47149.804* 2962 -0.010 3 Rhode (1990a) Figure 12. O-C diagram of SU Cas Figure 13. gamma-velocity values of SU Cas folded with the 462.5 day period Table 19. gamma-velocities of SU Cas J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 20229 162 -6.3 1.2 4 Adams & Shapley (1918) 21252 153 -8.0 1.2 4 Adams & Shapley (1918) 34307 17 -8.0 0.6 4 Abt (1959) 34621 23 -8.5 0.3 14 Abt (1959) 35051 1 -8.2 0.7 3 Abt (1959) 36451 1 -9.6 1.0 1 Abt (1959) 40943 47 -3.8 0.3 7 Niva & Schmidt (1979) 41962 29 -8.9 0.1 63 Gieren (1976) 43406 3 -7.5 0.1 27 Niva & Schmidt (1979) 43453 59 -6.4 1.2 12 Wilson et al. (1989) 43810 28 -5.7 0.1 51 Beavers & Eitter (1986) 44079 264 -5.6 0.8 23 Barnes et al. (1987) 44574 46 -4.2 0.2 14 Haupl (1988) 44895 54 -5.4 0.3 11 Haupl (1988) 46866 1 -4.9 0.4 2 Samus (1990) SZ Cassiopeiae The very rapid increase in the pulsation period of SZ Cas has been continuing (see Figure 14). The O-C diagram based on the photoelectric O-C residuals, however, results in an ephemeris slightly different from that determined in Paper III. The O-C residuals listed in Table 20 have been obtained using the new elements: C = 2443817.978 + 13.636857d*E (15) +-.029 +-.000305 The value of the period as a function of the epoch elapsed can be given as follows: P = 13.636857d + 18.72*E*10^-6 (16) +-.000305 +-1.37 Coker et al. (1989) obtained very accurate radial velocity measurements on SZ Cas, resulting in one of the best radial velocity Figure 14. Upper panel: O-C diagram of SZ Cas Lower panel: gamma-velocities for the same Cepheid Table 20. O-C residuals for SZ Cas Norm.max. E O-C W Reference JD2400000+ 36824.793 -513 2.523 3 Oosterhoff (1960) 36838.243 -512 2.336 3 Weaver et al. (1960) 36892.930 -508 2.475 3 Bahner et al. (1962) 43817.966 0 -0.012 3 Szabados (1981) 44404.379* 43 0.016 3 Moffett & Barnes (1984) 44963.567* 84 0.093 3 Moffett & Barnes (1984) 45672.744* 136 0.153 2 Berdnikov (1986) J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 26892 488 -43.8 1.8 7 Joy (1937) 46794 91 -46.5 0.1 24 Coker et al. (1989) 47024 47 -46.9 0.1 26 Coker et al. (1989) 47194 94 -46.3 0.1 20 Coker et al. (1989) Table 21. gamma-velocities of SZ Cas curves ever observed for a Cepheid. There is a hint that the y-velocity slightly varies from year to year (see Table 21), but further accurate measurements are necessary to confirm this suspicion. The recent study about the position of SZ Cas on the two-colour diagram (Usenko, 1990b) assumes a B3 - B4 companion to this Cepheid. BY Cassiopeiae The new version of the O-C diagram suggests an early phase jump in addition to the recent period change (see Table 22 and Figure 15). The pulsation of BY Cas can be characterized with the following periods during the various time intervals: between J.D. 2428500 and 2432000 P = 3.221315 +-.000112 days, between J.D. 2432000 and 2434500 P = 3.221557 +-.000104 days, between J.D. 2435500 and 2440000 P = 3.222618 +-.000037 days, after J.D. 2443000 P = 3.222199 +-.000031 days. The O-C residuals have been calculated with this latter period: C = 2441774.634 + 3.222199d*E (17) +-.019 +-.000031 The phase jump occurring at J.D. 2432000 was as large as 0.2 day. The type of the most recent period change is not clear yet. Further photometric observations are necessary. The available sporadic radial velocity observations (Joy, 1937; Scmtus, 1990) are not enough for the determination of the radial velocity curve itself. Nevertheless, BY Cas is a promising candidate for binarity: Usenko Table 22. O-C residuals for BY Cas Norm.max. E O-C W Reference JD2400000+ 28563.344 -4100 -0.274 1 Parenago (1940) 29223.824 -3895 -0.345 1 Kukarkina (1954) 30480.163 -3505 -0.664 1 Satyvaldiev (1970) 30650.693 -3452 -0.910 1 Dirks & Vaucouleurs (1949) 31014.735 -3339 -0.977 1 Satyvaldiev (1970) 31781.530 -3101 -1.065 1 Ashbrook (1954) 32048.696 -3018 -1.341 1 Dirks & Vaucouleurs (1949) 32132.390 -2992 -1.425 1 Satyvaldiev (1970) 33524.240 -2560 -1.565 1 Satyvaldiev (1970) 33736.708 -2494 -1.761 1 Kukarkina (1954) 33878.460 -2450 -1.786 1 Ashbrook (1954) 34361.768 -2300 -1.808 1 Kheilo (1962) 35557.258 -1929 -1.754 1 Kheilo (1962) 35615.515 -1911 -1.497 1 Satyvaldiev (1970) 36143.784 -1747 -1.668 1 Kheilo (1962) 36820.545 -1537 -1.569 3 Oosterhoff (1960) 36827.004 -1535 -1.555 3 Weaver et al. (1960) 36843.175 -1530 -1.495 1 Kheilo (1962) 36910.801 -1509 -1.535 3 Bahner et al. (1962) 38248.220 -1094 -1.328 1 Satyvaldiev (1970) 38409.256 -1044 -1.402 3 Malik (1965) 38660.818 -966 -1.172 1 Satyvaldiev (1970) 39785.406 -617 -1.131 3 Szabados (1977) 41774.189 0 -0.445 3 Szabados (1977) 43079.635 405 0.010 2 Szabados (1977) 43456.607* 522 -0.015 2 present paper 44168.732* 743 0.004 3 present paper Figure 15. O-C diagram of BY Cas (1990b) assumes a B5 photometric companion, thus supporting the earlier suspicion published by Kurochkin (1966) and Madore and Fernie (1980). The phase jump in the pulsation is a further evidence for duplicity. DD Cassiopeiae The normal light curve formed on the basis of the observations obtained by Moffett and Barnes (1984) made the re-discussion of the previous O-C diagram possible. According to the recent photoelectric observations (see Table 23 and Figure 16) a period change occurred between J.D. 2438000 and 2442500. The O-C residuals have been calculated with the ephemeris: C = 2442780.426 + 9.811656d*E (18) +-.009 +-.000060 The O-C residuals based on earlier photographic observations are compatible with the phase jump interpretation, because DD Cas was pulsating with practically the same period between J.D. 2430000 and 2438000, the phase jump being about 0.2 day. The light-time effect Figure 16. Upper panel: O-C diagram of DD Cas Lower panel: gamma-velocities for the same Cepheid Table 23. O-C residuals for DD Cas Norm.max. E O-C W Reference JD2400000+ 17063.619 -2621 -0.457 Parenago (1940) 28601.761 -1445 -0.822 Parenago (1940) 29170.967 -1387 -0.692 Parenago (1940) 30584.392 -1243 -0.146 Solov'yov (1958) 30976.882 -1203 -0.122 Solov'yov (1958) 36137.753 -677 -0.182 Makarenko (1969) 36490.636 -641 -0.519 Makarenko (1969) 36804.942 -609 -0.185 3 Oosterhoff (1960) 36843.862 -605 -0.512 Makarenko (1969) 36932.426 -596 -0.253 3 Bahner et al. (1962) 37197.343 -569 -0.251 Makarenko (1969) 37579.865 -530 -0.383 Makarenko (1969) 37923.516 -495 -0.140 Makarenko (1969) 38296.432 -457 -0.067 Makarenko (1969) 38688.932 -417 -0.033 Makarenko (1969) 42780.422 0 -0.004 3 Szabados (1980) 43388.760* 62 0.011 3 Chekhanikhina (1982) 44252.160* 150 -0.014 3 Moffett & Barnes (1984) 44958.635* 222 0.021 3 Moffett & Barnes (1984) 45125.371* 239 -0.041 1 present paper Table 24. gamma-velocities of DD Cas J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 26983 15 -56.5 4.5 2 Joy (1937) 27565 178 -69.5 2.3 5 Joy (1937) 28097 1 -73.3 4.5 1 Joy (1937) 44184 57 -67.5 1.6 7 Barnes et al. (1988) 44508 49 -70.6 1.3 10 Barnes et al. (1988) 44877 76 -69.1 2.0 5 Barnes et al. (1988) suggested in Paper II cannot be responsible for the shape of the O-C graph, since the amplitude of the wave would correspond to an enormously massive companion. Duplicity of DD Cas suggested by Madore (1977) and Madore and Fernie (1980) has been checked by the radial velocity measurements, too. As Table 24 and the lower panel of Figure 16 shows, the variation in the gamma-velocity is very probable, but further observations have to confirm the orbital motion. DL Cassiopeiae The new normal light curve based on the observations obtained by Moffett and Barnes (1984) defines the moment of light maxima more clearly than the previously used one. Owing to the new normal curve, a systematic correction of -0.192 day has been applied to the photoelectric O-C residuals published in Paper II. These corrected values, together with the more recent O-C residuals are listed in Table 25 and shown plotted in Figure 17. The new ephemeris for calculating the moments of maxima is as follows: C = 2442780.172 + 5.000598d*E (19) +-.011 +-.000022 The pulsation period of DL Cas has remained constant for the last decades, although Meyers (1988) determined a continuously increasing period. This latter study was based on photographic observations and, in my opinion, Meyers' (1988) O-C diagram can be better represented with two linear sections and a phase jump in between. The phase jump might occur at about J.D. 2429000, therefore unnoticeable in the O-C diagrams both in Paper II and here, in Figure 17. Table 25. O-C residuals for DL Cas Norm.max. E O-C W Reference JD2400000+ 36163.576 -827 -0.101 3 Arp et al. (1959) 36803.750 -747 0.025 3 Oosterhoff (1960) 37219.860 -695 0.104 2 Mitchell et al. (1964) 37947.796 -604 -0.015 2 Williams (1966) 38707.934 -509 0.066 2 Haug (1970) 42692.156* -11 -0.009 3 Szabados (1980) 43468.311* 86 0.088 1 Szabados (1980) 44292.283* 189 -0.002 3 Moffett & Barnes (1984) 44532.191* 219 -0.112 2 Eggen (1983a) 44972.332* 274 -0.004 3 Moffett & Barnes (1984) 45684.408* 363 0.019 3 Berdnikov (1986) 46284.450* 438 0.016 3 Berdnikov (1987) Figure 17. O-C diagram of DL Cas The spectroscopic binary nature of DL Cas was discovered quite recently, independently by two groups (Harris et al., 1987; Mermilliod et al., 1987). The orbital period is rather short, at least among the Cepheid binaries: 688.0 days (Harris et al., 1987). The orbital radial velocity curve gives an approximate value for the amplitude of the light-time effect in the O-C diagram. The full amplitude of this wave is about 0.02 day, therefore it can hardly be pointed out from the available photometric data. It is worth mentioning that DL Cas is one of the calibrating Cepheids for the period - luminosity relationship, because this Cepheid belongs to the open cluster NGC 129 (see Walker, 1987 and the references therein). IX Cassiopeiae Being a newly discovered spectroscopic binary (Harris and Welch, 1989), this Population II Cepheid would deserve more attention. The part of the O-C diagram based on photoelectric observations is shown in Figure 18 (see also Table 26). The frequent variations in the pulsation period are intrinsic to this star, and the straight line fit to the recent O-C residuals: C = 2442780.264 + 9.154549d*E (20) +-.030 +-.000104 does not necessarily mean constancy of the period. When determining the O-C residuals, a new normal curve based on the photometric observations made by Harris and Welch (1989) was used. The earlier O-C residuals have also been altered according to the new normal curve. The radial velocity measurements of IX Cas were published by Harris and Wallerstein (1984) and Harris and Welch (1989). Moreover, this latter paper also deals with the determination of the orbital parameters: the orbital period of IX Cas is 110.29 days. Figure 18. O-C diagram of IX Cas Table 26. O-C residuals for IX Cas Norm.max. E O-C W Reference JD2400000+ 36802.848 -653 0.504 3 Oosterhoff (1960) 36903.457 -642 0.413 3 Bahner et al. (1962) 37205.051 -609 -0.093 1 Mitchell et al. (1964) 37974.445 -525 0.319 1 Williams (1966) 42560.633* -24 0.078 2 Szabados (1980) 43247.037* 51 -0.109 2 Szabados (1980) 44034.452* 137 0.015 3 Harris & Welch (1989) 47348.390* 499 0.006 3 Harris & Welch (1989) V636 Cassiopeiae V636 Cas is one of the recently discovered Cepheid variables (Burki and Benz, 1982), therefore it does not have a long history of observations. The new photometric observations of this Cepheid listed in Table 108 are differential magnitudes with respect to BD +62 259. The O-C residuals in Table 27 and in Figure 19 (upper panel) have been calculated using the formula: C = 2444519.260 + 8.375735d*E (21) +-.009 +-.000039 The number of the existing radial velocity observational series is also small but it can be stated with certainty that the gamma-velocity of V636 Cas is varying. In addition to the gamma-velocity values listed in Table 28, there are two more series of observations with no information about the moment or epoch of the measurements. Redman (1930) published a single radial velocity value of -19 km/s, being more positive than any other velocity value published for this star. Boulon et al. (1958) gave -31 km/s as the average of seven measurements. This latter value is even more negative than the extreme gamma-velocity obtained from Samus' (1990) radial velocity data. Note that the value published by Boulon et al. is probably close to the gamma-velocity because of the short pulsation period and very low amplitude variation. However, the companion is not bright and blue enough to appear in the IUE spectrum of the Cepheid (Arellano Ferro and Madore, 1986). Table 27. O-C residuals for V636 Cas Norm.max. E O-C W Reference JD2400000+ 41964.655* -305 -0.006 3 Burki & Benz (1982) 44519.250* 0 -0.010 3 Burki & Benz (1982) 45231.237* 85 0.040 2 present paper 47794.140* 391 -0.032 1 present paper Figure 19. Upper panel: O-C diagram of V636 Cas Lower panel: gamma-velocities for the same Cepheid Table 28. gamma-velocities of V636 Cas J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 43855 42 -24.5 0.1 12 Burki & Benz (1982) 44201 26 -25.2 0.1 20 Burki & Henz (1982) 44558 57 -24.3 0.1 20 Burki & Benz (1982) 47282 266 -30.0 0.8 2 Samus (1990) IR Cephei The new photoelectric observations published here (see Table 108) confirm the value of the pulsation period determined in Paper I. This means that no new change has occurred in addition to that noted in Paper I (p. 43). The O-C residuals in Table 29 and in Figure 20 have been calculated using the equation: C = 2441696.581 + 2.114088d*E (22) +-.005 +-.000004 Table 29. O-C residuals for IR Cep Norm.max. E O-C W Reference JD2400000+ 40965.096 -346 -0.011 3 Wachmann (1976) 41696.580 0 -0.001 3 Szabados (1977) 43045.394 638 0.025 2 Szabados (1977) 47413.066* 2704 -0.009 2 present paper Figure 20. O-C diagram of IR Cep There is only one radial velocity measurement series on IR Cep (Samus, 1990), giving -4.9 km/s for the gamma-velocity. Although the membership of IR Cep in Cep OB2 association has been doubted on account of the age difference between the Cepheid and the association (Kun and Szabados, 1988), it should be noted that the gamma-velocity of IR Cephei derived here is in a good agreement with the radial velocity of some bona fide members of Cep OB2. For example, the bright O-star, HD 206267, the most massive member of the IC 1396 + Tr 37 complex, forming one part of Cep OB2 (Kun, 1986), has an average radial velocity of -8 km/s (Hoffleit and Jaschek, 1982). V351 Cephei This relatively new Cepheid variable has been observed frequently since the discovery of its light variability (see Table 30 and Figure 21). The pulsation period is considered to be constant during the interval of the photoelectric observations, although Erleksova (1978) was able to point out two major period changes on the basis of archival photographic observations. The O-C residuals have been calculated here using a new Figure 21. O-C diagram of V351 Cep Table 30. O-C residuals for V351 Cep Norm.max. E O-C W Reference JD2400000+ 42030.985 -230 -0.006 2 Percy (1975) 42676.380 0 -0.003 3 Szabados (1977) 42993.459* 113 -0.008 2 Szabados (1977) 43700.608* 365 0.016 3 Henden (1979) 44071.005* 497 0.014 3 Diethelm & Tammann (1982) 44528.380* 660 0.003 2 Eggen (1985) 44696.720* 720 -0.020 3 Arellano Ferro (1984) normal light curve based on Arellano Ferro's (1984) observations and the following ephemeris: C = 2442676.383 + 2.806052d*E (23) +-.005 +-.000010 Unfortunately no radial velocity measurements have been made on this Cepheid so far. X Cygni Although X Cygni belongs to the most frequently observed Cepheids, its O-C diagram has not yet been interpreted concordantly. The most comprehensive analysis has been performed by Evans (1984). In the present paper, however, I propose a new interpretation, viz. a phase jump in the O-C diagram, that has not been mentioned in the literature on X Cygni so Figure 22. Upper panel: O-C diagram of X Cyg Lower panel: gamma-velocities for the same Cepheid Table 31. O-C residuals for X Cyg Norm.max. E O-C W Reference JD2400000+ 17055.372 -1634 -0.658 1 Wilkens (1906) 17694.729 -1595 -0.343 1 Jordan (1919) 23183.584 -1260 -0.695 1 Henroteau (1924) 25117.137 -1142 -0.654 1 Hellerich (1935) 26804.859 -1039 -0.658 1 Kox (1935) 27148.928 -1018 -0.689 1 Dziewulski (1948) 27328.968 -1007 -0.891 1 Liau (1935) 28672.816 -925 -0.670 1 Dziewulski (1948) 33031.783 -659 -0.297 3 Eggen (1951) 33605.249 -624 -0.330 1 Romano (1951) 34605.093 -563 -0.013 3 Szabados (1981) 36096.180 -472 -0.024 1 Svolopoulos (1960) 37226.867 -403 0.050 3 Mitchell et al. (1964) 37898.718 -362 0.088 3 Williams (1966) 38881.767 -302 -0.005 3 Wisniewski & Johnson (1968) 39782.931 -247 -0.054 3 Szabados (1981) 40585.795* -198 -0.089 2 Feltz & McNamara (1980) 40929.879 -177 -0.105 2 Evans (1976) 41618.198 -135 0.015 3 Landis (1973) 43486.127* -21 -0.024 3 Moffett & Barnes (1984) 43830.274 0 0.023 3 Szabados (1981) 44092.421* 16 -0.001 3 Moffett & Barnes (1984) 44534.885* 43 0.049 2 Eggen (1983b) 45534.385* 104 0.022 3 Berdnikov (1986) Table 32. gamma-velocities of X Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 22094 191 9.3 0.5 23 Duncan (1921) 22973 1 7.0 3.0 1 Harper (1934) 35332 21 9.5 0.2 17 Abt (1978) 40945 36 10.1 1.1 3 Schmidt (1974) 40952 169 8.2 0.3 5 Evans (1976) 43329 150 5.6 0.3 8 Beavers & Eitter (1986) 43393 49 4.2 1.1 15 Wilson et al. (1989) 44013 271 9.2 0.7 27 Barnes et al. (1987) 44055 382 7.9 0.5 18 Wallerstein (1983) 47473 1 13.9 0.7 1 Samus (1990) far. The photoelectric O-C residuals supplemented with the early photographic ones are listed in Table 31. After J.D. 2434000 the photoelectric O-C residuals can be best represented by a straight line as follows: C = 2443830.251 + 16.385692d*E (24) +-.011 +-.000041 As one can see in Figure 22 (the two residuals from the epoch earlier than J.D. 2420000 have not been plotted here), the O-C residuals before J.D. 2429000 define a slightly shorter, though constant period: P = 16.385356 +- 0.000176 days. According to this interpretation the phase jump with an amplitude of 0.8 day occurred between J.D. 2429000 and 2434000. The gamma-velocity values of X Cyg are listed in Table 32 and shown plotted in the lower panel of Figure 22. The variation in the gamma-velocity seems to be larger than determined by Evans (1984) but its physical reality cannot be supported with definitive evidence for duplicity. The phase jump interpretation of the O-C diagram; however, implies the duplicity of X Cyg because such phenomena only occur in binary Cepheids. SU Cygni The phase jump reported in Paper I is confirmed here. Because the phase jump in the pulsation period of SU Cyg seems to be the best documented example for this phenomenon, the whole O-C diagram has been studied again (including the O-C diagram for the median brightness), only omitting the very uncertain visual observations before the jump and all visual data after the phase jump. The O-C residuals corresponding to the maximum light, listed in Table 33, have been calculated with the current ephemeris: Cmax = 2441778.977 + 3.845512d*E (25) +-.004 +-.000004 while those for the median brightness (see Table 34) have been computed as follows: Cmed = 2441778.616 + 3.845500d*E (26) +-.005 +-.000010 Figure 23. Upper panel: O-C diagram of SU Cyg (maximum brightness) Lower panel: the same for the median brightness Table 33. O-C residuals for SU Cyg (maximum brightness) Norm.max. E O-C W Reference JD2400000+ 14256.621 -7157 -0.027 1 Muller & Kempf (1897) 14491.161 -7096 -0.063 1 Zinner (1932) 14564.221 -7077 -0.067 1 Luizet (1899) 14591.118 -7070 -0.089 1 Wendell (1913) 17052.191 -6430 -0.144 2 Wilkens (1906) 17829.048 -6228 -0.080 1 Zeipel (1908) 17882.822 -6214 -0.143 1 van der Bilt (1925) 18175.117 -6138 -0.107 1 van der Bilt (1925) 18528.908 -6046 -0.103 1 van der Bilt (1925) 19271.104 -5853 -0.091 1 van der Bilt (1925) 21086.203 -5381 -0.074 1 Luyten (1922) 21443.751 -5288 -0.159 1 Luyten (1922) 21943.616 -5158 -0.210 1 Luyten (1922) 23320.515 -4800 -0.004 1 Hellerich (1925) 23662.684 -4711 -0.086 1 Hellerich (1925) 24028.020 -4616 -0.074 1 Hellerich (1925) 25100.868 -4337 -0.123 2 Hellerich (1935) 25696.940 -4182 -0.106 1 Zverev (1936) 26423.739 -3993 -0.109 1 Zverev (1936) 26923.669 -3863 -0.095 1 Florya & Kukarkina (1953) 27277.410 -3771 -0.141 1 Florya & Kukarkina (1953) 27677.322 -3667 -0.162 1 Krebs (1935) 28050.410 -3570 -0.089 1 Krebs (1936) 33126.659 -2250 0.084 3 Eggen (1951) 33680.364 -2106 0.035 1 Chuprina (1952) 34368.640 -1927 -0.035 1 Shteiman (1958) 34591.666 -1869 -0.049 2 Szabados (1977) 35356.949 -1670 -0.023 2 Walraven et al. (1958) 35645.305 -1595 -0.080 1 Shteiman (1958) 36099.119 -1477 -0.037 2 Svolopoulos (1960) 37172.013* -1198 -0.041 2 Mitchell et al. (1964) 37498.922* -1113 0.000 2 Mitchell et al. (1964) 37941.117 -998 -0.039 2 Williams (1966) 38664.105* -810 -0.007 2 Wisniewski & Johnson (1968) 39029.433* -715 -0.003 2 Wisniewski & Johnson (1968) 40452.240* -345 -0.035 2 Feltz & McNamara (1980) 40867.591* -237 0.000 3 Evans (1976) 41225.237* -144 0.014 3 Feltz & McNamara (1980) 41540.552* -62 -0.003 2 Szabados (1977) 41932.815* 40 0.018 2 Szabados (1977) 43344.084* 407 -0.016 3 Moffett & Barnes (1984) 43367.170* 413 -0.003 3 Fernie (1979b) 43378.722* 416 0.012 2 present paper 43786.320* 522 -0.014 3 Moffett & Barnes (1984) 44474.659* 701 -0.022 2 Berdnikov & Bogdanov (1987) 44866.935* 803 0.012 3 present paper 46412.858* 1205 0.039 3 present paper 47793.366* 1564 0.008 2 present paper The difference between the periods as determined from the various parts of the O-C diagram (see Figure 23) is insignificant: for the maximum brightness: before the phase jump P = 3.845502 +- 0.000007 days after the phase jump P = 3.845512 +- 0.000004 days, Table 34. O-C residuals for SU Cyg (median brightness) Norm.max. E O-C W Reference JD2400000+ 14590.722 -7070 -0.209 1 Wendell (1913) 17051.794 -6430 -0.257 2 Wilkens (1906) 17828.641 -6228 -0.201 1 Zeipel (1908) 24738.969 -4431 -0.237 1 Moncibowitz (1938) 25100.518 -4337 -0.165 2 Hellerich (1935) 25131.234 -4329 -0.213 1 Moncibowitz (1938) 25323.499 -4279 -0.223 1 Moncibowitz (1938) 25696.552 -4182 -0.183 1 Zverev (1936) 27277.010 -3771 -0.226 1 Florya & Kukarkina (1953) 27676.857 -3667 -0.311 1 Krebs (1935) 41778.619 0 0.003 3 Szabados (1977) 43343.719* 407 -0.016 3 Moffett & Barnes (1984) 43366.817* 413 0.010 3 Fernie (1979b) 44866.555* 803 0.003 3 present paper for the median brightness: before the phase jump P = 3.845502 +- 0.000011 days after the phase jump P = 3.845500 +- 0.000010 days. (Note that the O-C residuals before J.D. 2420000 listed in the respective tables do not appear in Figure 23.) The amount of the phase jump, however, clearly differs if the two O-C diagrams are compared. The phase jump is 0.13 day for the maximum brightness, while the value of 0.21 day can be determined from the moments of the median brightness. This difference is a manifestation of a noticeable change in the shape of the light curve, in the sense that the ascending branch has become steeper since the phase jump. The moment of the phase jump is not known yet: it might occur between J.D. 2428000 and 2433000. SU Cyg belongs to the most thoroughly studied Cepheids from the spectroscopic point of view, as well. The spectroscopic orbit was recently published by Evans (1988), while the detailed study of the companion (a spectroscopic binary itself) was performed by Evans and Bolton (1990). The orbital period is too short (549.16 days) to cause noticeable light- time effect in the O-C diagram. SZ Cygni The new version of the O-C diagram (see Table 35 and Figure 24) is interpreted as one more case for a phase jump. The earlier suggestion on the light-time effect (see Paper III) is not tenable any more because the amplitude of the O-C wave would involve much too large orbital velocity variations that are not observed (see below). The O-C residuals have been calculated using the ephemeris: C = 2443760.344 + 15.110228d*E (27) +-.033 +-.000078 This period describes well the behaviour of the pulsation after J.D. 2430000, while before that epoch the period was 15.110536 +- 0.000161 Table 35. O-C residuals for SZ Cyg Norm.max. E O-C W Reference JD2400000+ 14991.044 -1904 0.574 1 Williams (1900) 15973.353 -1839 0.718 1 Florya & Parenago (1933) 17499.452 -1738 0.684 1 Florya & Parenago (1933) 22667.246 -1396 0.780 1 Henroteau (1924) 30659.819 -867 0.043 1 Filin (1951) 31702.567 -798 0.185 1 Filin (1951) 32201.239 -765 0.219 1 Kulikov (1957) 32820.760 -724 0.221 1 Filin (1951) 34588.194 -607 -0.241 1 Kulikov (1957) 35389.154 -554 -0.124 1 Kulikov (1957) 36779.369 -462 -0.050 3 Oosterhoff (1960) 36794.540 -461 0.011 3 Weaver et al. (1960) 37217.632 -433 0.017 2 Mitchell et al. (1964) 37942.574 -385 -0.332 1 Girnyak (1971) 38229.979 -366 -0.022 3 Kwee & Braun (1967) 39302.479 -295 -0.348 1 Girnyak (1971) 43760.486 0 0.142 3 Szabados (1981) 44349.673* 39 0.030 3 Moffett & Barnes (1984) 44999.388* 82 0.005 3 Moffett & Barnes (1984) 45603.789* 122 -0.003 3 Berdnikov (1986) Figure 24. Upper panel: O-C diagram of SZ Cyg Lower panel: gamma-velocities for the same Cepheid Table 36. gamma-velocities of SZ Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 24819 962 -8.9 3.2 3 Joy (1937) 26435 231 -18.7 2.3 5 Joy (1937) 31304 7 -12.2 0.8 17 Struve (1945) 44529 309 -10.7 0.9 21 Barnes et al. (1988) days. The phase jump was as large as 0.8 day (or about 0.05 pulsation period). The visual observations also support the occurrence of a phase jump at about J.D. 2430000. Nevertheless, these low quality observations have not been taken into account in the line fitting procedure. The analysis of the available radial velocity data leads to the conclusion that the gamma-velocity of SZ Cyg is variable (see Table 36) but further spectroscopic observations are desirable in order to point out definitely the effect of the hypothetical B4 photometric companion (Madore, 1977). TX Cygni The recent photoelectric observations clearly show the major period change suspected in Paper III. Both the recent value of the pulsation period and the moment of the sudden increase can be determined from the available data. The O-C residuals listed in Table 37 (see also Figure 25) can be represented with the elements: C = 2443795.007 + 14.711635d*E (28) +-.019 +-.000272 Figure 25. Upper panel: O-C diagram of TX Cyg Lower panel: gamma-velocities for the same Cepheid Table 37. O-C residuals for TX Cyg Norm.max. E O-C W Reference JD2400000+ 43794.971 0 -0.036 3 Szabados (1981) 44339.391* 37 0.054 3 Moffett & Barnes (1984) 45001.361* 82 0.000 3 Moffett & Barnes (1984) 45545.668* 119 -0.024 2 Berdnikov (1986) Table 38. gamma-velocities of TX Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 26188 1256 -24.4 1.8 7 Joy (1937) 31306 6 -22.5 0.8 14 Struve (1945) 43741 1 -19.6 0.3 1 Harris et al. (1979) Combining the above elements with those published in Paper III, the change in the period occurred at about J.D. 2440500. Since the period valid previously was 14.708157 days, the difference (0.024 per cent) is unusually large for a classical Cepheid. It is interesting to note that Kovacs et al. (1990) determined a value of 14.1369 days for the period on the basis of the available radial velocity data. None of the recent photoelectric observational series, however, supports a period as short as this. The gamma-velocity of TX Cyg may be variable (see Table 38) but further observations are necessary to confirm this suspicion. VZ Cygni Figure 26. Upper panel: O-C diagram of VZ Cyg Lower panel: gamma-velocities for the same Cepheid The plot of O-C residuals supplemented with the data derived from the recently published photoelectric observations (see Table 39 and Figure 26) can best be represented with a parabola, indicating a continuous period decrease. The O-C residuals have been calculated as follows: C = 2441705.702+ 4.864450d*E (12) +-.007 +-.000008 The value of the pulsation period can be calculated using the formula: P = 4.864450d - 4.55*10^-8*E (30) +-.000008 +-.44 Table 39. O-C residuals for VZ Cyg Norm.max. E O-C W Reference JD2400000+ 20627.598 -4333 -0.442 1 Doberck (1920) 21114.079 -4233 -0.406 2 Jordan (1929) 21498.432 -4154 -0.345 1 Doberck (1920) 22179.388 -4014 -0.412 1 Doberck (1920) 22656.134 -3916 -0.382 1 Jordan (1929) 23507.509 -3741 -0.286 1 Nielsen (1954) 24996.109 -3435 -0.207 1 Wachmann (1935) 26163.550 -3195 -0.234 1 Wachmann (1935) 26513.862 -3123 -0.163 1 Wachmann (1935) 26898.035 -3044 -0.281 1 Wachmann (1935) 27321.296 -2957 -0.227 2 Gesundheit (1938) 27739.687 -2871 -0.179 2 Gesundheit (1938) 28848.680 -2643 -0.281 1 Abidov (1963) 29096.894 -2592 -0.154 1 Abidov (1963) 29452.007 -2519 -0.145 1 Abidov (1963) 29812.004 -2445 -0.118 1 Abidov (1963) 32755.060 -1840 -0.054 1 Novikov (1951) 33387.402 -1710 -0.091 1 Abidov (1963) 34219.193 -1539 -0.120 1 Abidov (1963) 34589.022 -1463 0.010 1 Abidov (1963) 35362.501 -1304 0.042 1 Vyskupaitis (1961) 35732.104* -1228 -0.053 3 Bahner & Mavridis (1977) 36106.672* -1151 -0.048 3 Bahner & Mavridis (1977) 36773.146 -1014 -0.004 3 Weaver et al. (1960) 36802.348 -1008 0.012 3 Oosterhoff (1960) 37352.009 -895 -0.010 3 Mitchell et al. (1964) 41160.858* -112 -0.026 2 Feltz & McNamara (1980) 41525.716* -37 -0.001 2 Feltz & McNamara (1980) 41705.698 0 -0.004 3 Szabados (1977) 43062.886 279 0.002 2 Szabados (1977) 44366.557* 547 0.001 3 Moffett & Barnes (1984) 44911.356* 659 -0.019 3 Moffett & Barnes (1984) Table 40. gamma-velocities of VZ Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 24235 949 -19.4 3.2 3 Joy (1937) 25819 37 -14.4 2.6 4 Joy (1937) 44489 241 -18.5 1.8 6 Barnes et al. (1988) 45342 1 -21.0 2.8 3 Barnes et al. (1988) Variations in the gamma-velocity of VZ Cyg cannot be excluded (see Table 40) but further radial velocity measurements are necessary to make a firm statement on this matter. BZ Cygni The pulsation period keeps on being constant, but its value is slightly modified with respect to that published in Paper III. The O-C residuals listed in Table 41 and plotted in Figure 27 can be approximated with a line: C = 2443774.199 + 10.142222d*E (31) +-.032 +-.000065 As to the radial velocity measurements of BZ Cyg, a difference as large as 20 km/s can be seen between the gamma-velocity of the available radial velocity measurement series (see Table 42 and the lower panel of Figure 27. Upper panel: O-C diagram of BZ Cyg Lower panel: gamma-velocities for the same cepheid Table 41. O-C residuals for BZ Cyg Norm.max. E O-C W Reference JD2400000+ 36786.104 -689 -0.104 3 Weaver et al. (1960) 36796.408 -688 0.058 3 Oosterhoff (1960) 37273.217 -641 0.182 1 Mitchell et al. (1964) 43013.533* -75 0.001 2 Chekhanikhina (1982) 43774.144 0 -0.055 3 Szabados (1981) 45539.006* 174 0.060 2 Berdnikov (1986) Table 42. gamma-velocities of BZ Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 28000 312 -21.4 1.8 7 Joy (1937) 31306 6 -6.4 0.8 14 Struve (1945) 47473 1 -27.0 0.6 1 Samus (1990) Figure 27). Since BZ Cyg is a new spectroscopic binary beyond doubt, this Cepheid deserves immediate attention of the spectroscopists. DT Cygni This bright Cepheid was frequently observed photoelectrically during the last two decades, therefore the most recent part of the O-C diagram (see Table 43 and Figure 28) is of exceptionally good quality. Due to the new normal light curve based on the observations obtained by Moffett and Barnes (1984), a correction of -0.080 day has been applied to the previously determined O-C residuals (taken from Paper I). The section of the O-C diagram between J.D. 2440000 and 2447000 can be approximated with the line: C = 2441737.702 + 2.499086d*E (32) +-.005 +-.000005 Figure 28. Upper panel: O-C diagram of DT Cyg Lower panel: gamma-velocities for the same Cepheid Table 43. O-C residuals for DT Cyg Norm.max. E O-C W Reference JD2400000+ 24375.503 -6947 -1.049 3 Huffer (1928b) 24695.458 -6819 -0.977 2 Huffer (1928b) 27546.928 -5678 -0.964 2 Schneller (1936) 32975.235 -3506 -0.667 2 Eggen (1951) 36099.447 -2256 -0.317 2 Svolopoulos (1960) 37176.620 -1825 -0.250 3 Mitchell et al. (1964) 37579.011 -1664 -0.212 3 Johansen (1971) 38496.161 -1297 -0.226 3 Johansen (1971) 38871.042 -1147 -0.208 3 Wisniewski & Johnson (1968) 40185.742* -621 -0.028 1 Feltz & McNamara (1980) 41297.846* -176 -0.017 2 Feltz & McNamara (1980) 41737.718 0 0.016 3 Szabados (1977) 43044.725 523 0.001 2 Szabados (1977) 43379.606* 657 0.004 3 Moffett & Barnes (1984) 43399.611* 665 0.017 3 present paper 43754.468* 807 0.004 2 present paper 43871.925* 854 0.004 3 Moffett & Barnes (1984) 44149.328* 965 0.008 3 present paper 44534.159* 1119 -0.020 1 Eggen (1985) 44684.129* 1179 0.005 2 present paper 44869.031* 1253 -0.026 2 Arellano Ferro (1984) 45646.256* 1564 -0.017 3 Guetter & Hewitt (1984) 46341.053* 1842 0.035 1 "Carlsberg" (1989) 47343.284* 2243 0.132 3 Rhode (1990b) Two other linear sections are also marked in the O-C graph in Figure 28, viz.: between J.D. 2424000 and 2428000 P = 2.499137 +- 0.000022 days, and between J.D. 2437000 and 2439000 P = 2.499101 +- 0.000023 days. Between the intervals of pulsating with the periods listed above, phase jumps caused some shifts in the O-C diagram. Since the visual and the photographic observations have not been taken into account in this study, only the last phase jump is seen well, the amount of the jump being about 0.2 day. The suggestion put forward in Paper I (p. 49) concerning the regular phase shift (about 0.22 day or its multiple) cannot be confirmed here, its verification should wait until the next occurrence of the phase jump. A new period change event can be suspected at the most recent O-C residual in Figure 28. Further observations will clarify whether this change will turn out to be a new phase shift. In any case, the prediction for the moment of the light maximum near J.D. 2450000 in Table 110 may not be accurate. The phase jump is a characteristic feature of the binary Cepheids. There are a few pieces of evidence concerning the duplicity of DT Cygni. Most of them are summarized by Leonard and Turner (1986), concluding that the existence of an early type companion is uncertain. However, according Table 44. gamma-velocities of DT Cyg J.D. sigma v gamma sigma n Rererence 2400000+ [d] [km/s] [km/s] 20080 29 -3.3 2.1 3 Sanford (1930) 26865 93 -2.5 0.9 12 Sanford (1930) 28519 446 -2.5 2.1 3 Young (1939) 31905 284 0.0 0.4 7 Sanford (1951) 33486 25 -3.8 0.3 11 Grassberger & Herbig (1952) 33843 24 -4.1 0.4 9 Grassberger & Herbig (1952) 43392 43 -1.7 0.6 46 Wilson et al. (1989) 43711 206 -1.7 0.2 15 Beavers & Eitter (1986) 43946 218 -0.9 0.8 23 Barnes et al. (1987) 44832 2 -1.3 0.7 6 Arellano Ferro (1984) 45270 1 -2.4 2.3 1 present paper to a very recent paper by Usenko (1990b), DT Cyg has an A2-A3 type photometric companion. An early evidence for the changes in the gamma-velocity was published by Lloyd Evans (1968). His conclusion is confirmed here (see Table 44 and Figure 28). The extreme values of the gamma-velocity differ from each other by more than four km/s, and this difference exceeds the observational uncertainty. New high quality radial velocity measurements will hopefully solve the problem of duplicity of DT Cygni because the amplitude of the expected gamma-velocity changes is rather low. MW Cygni The two recent O-C residuals (see Table 45 and Figure 29) indicate that the pulsation period is slightly longer than determined in Paper II. The O-C graph can be approximated with a line as follows: C = 2442923.907 + 5.954666d*E (33) +-.007 +-.000007 There is an obvious variation in the gamma-velocity of MW Cyg. According to the data listed in Table 46, Moffett and Barnes (1987) underestimated Table 45. O-C residuals for MW Cyg Norm.max. E O-C W Reference JD2400000+ 30532.222 -2081 -0.025 1 Solov'yov (1946) 31038.435 -1996 0.041 1 Solov'yov (1946) 33884.781 -1518 0.057 1 Shteiman (1958) 34539.757 -1408 0.020 1 Shteiman (1958) 35677.074 -1217 -0.004 1 Shteiman (1958) 36802.499 -1028 -0.011 3 Oosterhoff (1960) 36820.339 -1025 -0.035 3 Weaver et al. (1960) 42923.911 0 0.004 3 Szabados (1980) 44364.932* 242 -0.004 3 Moffett & Barnes (1984) 44877.050* 328 0.013 3 Moffett & Barnes (1984) Figure 29. Upper panel: O-C diagram of MW Cyg Lower panel: gamma-velocities for the same Cepheid Table 46. gamma-velocities of MW Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 27487 311 -11.0 1.7 8 Joy (1937) 31304 7 -15.5 0.8 15 Struve (1945) 44514 282 -20.6 1.8 6 Barnes et al. (1988) the difference between the gamma-velocities determined from Joy's (1937) and their own data. While Moffett and Barnes derived -2.8 km/s, now the difference is -9.4 km/s. The deviating result is probably caused by the different methods in determining the gamma-velocities. In the present study the normal radial velocity curve has been based on Struve's (1945) measurements, and the other radial velocity measurements have been fitted to this normal curve. MW Cyg is therefore a new spectroscopic binary Cepheid candidate. V386 Cygni The O-C diagram (see Table 47 and Figure 30) confirms the earlier conclusion (in Paper II) about the constancy of the pulsation period of V386 Cygni. The new, slightly modified ephemeris for the moments of maxima is as follows: C = 2442777.141 + 5.257635d*E (34) +-.003 +-.000004 Figure 30. Upper panel: O-C diagram of V386 Cyg Lower panel: gamma-velocities for the same Cepheid Table 47. O-C residuals for V386 Cyg Norm.max. E O-C W Reference JD2400000+ 36762.403 -1144 -0.004 3 Weaver et al. (1960) 36804.456 -1136 -0.012 3 Oosterhoff (1960) 37251.385 -1051 0.018 2 Mitchell et al. (1964) 42777.160 0 0.019 3 Szabados (1980) 44359.691* 301 0.002 3 Moffett & Barnes (1984) 44927.497* 409 -0.017 3 Moffett & Barnes (1984) 46289.243* 668 0.002 3 Berdnikov (1987) Table 48. gamma-velocities of V386 Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 28571 90 -17.9 2.6 4 Joy (1937) 31006 7 -4.2 0.8 16 Struve (1945) 44581 257 -6.5 0.9 19 Barnes et al. (1988) 47010 15 -5.6 0.3 8 Metzger et al. (1990) The study of the gamma-velocity gives a new piece of evidence for the binary nature of V386 Cyg. It is noteworthy that there is a difference as large as 13.7 km/s between the gamma-velocities on a time-base of about 2500 days (see Table 48 and Figure 30, lower panel). Although Moffett and Barnes (1987) gave a smaller value for the discrepancy between the individual gamma-velocity values, the other pieces of evidence (Kurochkin, 1966; Madore, 1977; Madore and Fernie, 1980) make the spectroscopic binary interpretation very reasonable. V532 Cygni The new photoelectric observations confirm the occurrence of the phase jump first suggested in Paper I. Because a new normal light curve was used here, the previously determined normal maxima have been corrected by 0.102 day accordingly (see Table 49 and Figure 31). The O-C residuals have been calculated with the elements: C = 2441706.686 + 3.283494d*E (35) +-.007 +-.000007 Table 49. O-C residuals for V532 Cyg Norm.max. E O-C W Reference JD2400000+ 33889.249 -2381 0.562 1 Shteiman (1958) 34434.386 -2215 0.639 1 Shteiman (1958) 35642.593 -1847 0.520 1 Shteiman (1958) 36817.805 -1489 0.242 3 Oosterhoff (1960) 37684.512* -1225 0.106 1 Girnyak (1971) 38229.434 -1059 -0.032 3 Kwee & Braun (1967) 38439.599* -995 -0.010 1 Girnyak (1971) 39024.197* -817 0.126 1 Girnyak (1971) 39411.594* -699 0.070 1 Girnyak (1971) 41338.896* -112 -0.039 2 Feltz & McNamara (1980) 41706.661 0 -0.025 3 Szabados (1977) 43026.673 402 0.022 2 Szabados (1977) 43420.683* 522 0.013 3 present paper 44149.609* 744 0.003 2 present paper 44438.557* 832 0.004 3 Moffett & Barnes (1984) 44911.371* 976 -0.005 3 Moffett & Barnes (1984) 45009.840* 1006 -0.041 2 present paper 46490.739* 1457 0.002 2 present paper 47534.898* 1775 0.010 3 present paper Figure 31. O-C diagram of V532 Cyg Figure 32. Radial velocity observations of V532 Cygni folded with the 3.283494 day pulsation period. Zero phase is chosen arbitrarily at J.D. 2440000. Open circles: Barnes et al.'s (1988) data, filled circles: Metzger et al.'s (1990) observations This formula can be used for predicting the maxima after J.D. 2438000. Before that epoch a phase shift occurred: between J.D. 2431200 and 2435700 the pulsation period was 3.283514 +- 0.000056 days. This latter period was determined on the basis of the three O-C residuals listed in Table 49 supplemented with four other, less reliable O-C residuals taken from Paper I. These low quality residuals have not been listed nor plotted here. The phase difference, that can be seen between J.D. 2435700 and 2438000 is about 0.58 day (= 0.18 pulsation period). Because the phase jump is a characteristic feature of the binary Cepheids, one expects that V532 Cygni also belongs to a binary system. In addition to the previous photometric evidence for a B5 companion (Madore, 1977), in a more recent paper Usenko (1990b) also assumes a B7 - B8 photometric companion. The available radial velocity measurements are not enough for the numerical determination of the gamma-velocity itself, but the composite phase diagram of the two available radial velocity series indicates that a shift in the gamma-velocity might occur between the epochs of observation of the respective series. In Figure 32 open circles denote the data obtained by Barnes et al. (1988), while Metzger et al.'s (1990) velocity measurements are plotted as filled circles. Zero phase is arbitrarily chosen at J.D. 2440000, and the data are folded with the period given in Eq.(35). It is very unfortunate that Metzger et al. could not cover the phases of least negative velocities, because the systematic difference between the most negative values of the two radial velocity series is clearly seen. Further photometry and spectroscopy of V532 Cygni is extremely important. V924 Cygni This very low amplitude Cepheid continues to be a rather neglected variable. The O-C residuals based on the photoelectric observations published in the literature are listed in Table 50. These residuals have been calculated using the elements: C = 2443065.993 + 5.571305d*E (36) +-.017 +-.000044 In addition to the photoelectric O-C residuals, and the line fitted to these residuals, Figure 33 also shows another section of line corresponding to the O-C residuals from earlier epochs. Those rather inaccurate photographic O-C residuals are listed in Paper II, where the deviating part was explained as a systematic difference caused by the fact that the two sections had been obtained by different methods (because the early photographic observations have been unpublished, and the originally published normal maxima were used in Paper II). There is, however, another possibility, viz. the phase jump interpretation of the O-C diagram. The former pulsation period was 5.571106 +- 0.000029 days. Figure 33. O-C diagram of V924 Cyg Table 50. O-C residuals for V924 Cyg Norm.max. E O-C W Reference JD2400000+ 38558.778 -809 -0.029 2 Eggen (1969) 41260.881 -324 -0.009 3 Wachmann (1976) 43066.098 0 0.105 3 Szabados (1980) 44391.902* 238 -0.062 3 Moffett & Barnes (1984) 44893.368* 328 -0.013 3 Moffett & Barnes (1984) Again, the phase jump implies the existence of a companion. As to duplicity of V924 Cygni, the only evidence is the assumption of a B8 - B9 photometric companion by Usenko (1990b). It is worth mentioning, however, that V924 Cyg is one of the lowest amplitude Cepheids, and the extremely low amplitude light variability can well be explained with the photometric effect caused by the extra (and constant) light of a secondary star. Unfortunately, no radial velocity measurements of this interesting Cepheid have been performed yet. V1334 Cygni Arellano Ferro's (1984) photoelectric light curve served as the new normal curve for determining the moments of the normal maxima. The O-C residuals listed in Table 51 and plotted in Figure 34 have been calculated with the elements: C = 2441760.896 + 3.332804d*E (37) +-.018 +-.000024 V1334 Cygni has long been known as a visual binary (ADS 14859). Now there is growing evidence, including this paper, that the Cepheid component of the visual pair is itself a spectroscopic binary. As to the visual binary, Abt and Levy (1970) determined an orbital period of about 30 years from position angle measurements, while Henriksson (1982) derived an 80 year orbital period in the same manner. The gamma-velocity data of V1334 Cyg listed in Table 52, shown plotted in the lower panel of Figure Figure 34. Upper panel: O-C diagram of V1334 Cyg Lower panel: gamma-velocities for the same Cepheid Table 51. O-C residuals for V1334 Cyg Norm.max. E O-C W Reference JD2400000+ 40117.791 -493 -0.033 3 Millis (1969) 41761.010 0 0.114 3 Szabados (1977) 43657.233* 569 -0.028 3 Henden (1979) 43720.554* 588 -0.031 3 Percy et al. (1979) 44480.439* 816 -0.025 1 Parsons & Montemayor (1982) 44530.424* 831 -0.032 3 Bartolini et al. (1981) 44743.717* 895 -0.039 3 Arellano Ferro (1984) 46726.861* 1490 0.087 2 Arellano Ferro et al. (1987) Table 52. gamma-velocities of V1334 Cyg J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 26580 5 -2.4 2.1 3 Harper (1934) 27666 7 -4.2 3.0 2 Harper (1937) 40449 35 8.4 0.2 10 Abt & Levy (1970) 40551 23 8.3 0.2 10 Abt & Levy (1970) 44536 23 8.7 0.8 5 Arellano Ferro (1984) 44672 108 3.5 1.0 2 Parsons (1983) 44747 17 -1.5 0.8 5 Arellano Ferro (1984) 44831 6 -2.5 0.5 9 Arellano Ferro (1984) 34 clearly demonstrate that the gamma-velocity varies on a much shorter time-scale. This variation was already reported by Abt and Levy (1970) and Henriksson (1982), but now an upper limit can be deduced for the spectroscopic orbital period. A formal period search routine applied to the data set resulted in 1240 days as the most reliable value of the orbital period, but a number of other values between 500 and 1150 days would also give a resonable preliminary orbit. The 4.5 year long orbital period determined by Henriksson seems to be too long but the orbital velocity amplitude derived by him (23 km/s) is confirmed here, based on the data listed in Table 52, and one more value of the gamma-velocity, not reported in that table, because Shajn and Albitzky (1932) published -15.2 km/s as the average radial velocity of V1334 Cyg without reporting the epoch of the observation. The close companion to V1334 Cyg is an early type star: Henriksson derived a spectral type of B8III from the ultraviolet spectrum, while Usenko (1990b) assumes a B4 photometric companion. In the latest edition of the catalog of interferometric measurements of binary stars McAlister and Hartkopf (1988) reported that Yu. Balega had been able to separate the two components of the close pair at an angular distance of 0.035 arcsecond (epoch 1986.66). The shorter orbital period is expected to cause a light-time effect that cannot be pointed out in the O-C diagram, but the same statement is not valid a priori for the orbital motion of the visual pair. Therefore regular photometric and spectroscopic observations are recommended. V1726 Cygni There are only two series of photoelectric observations on this Cepheid, variability of which was only discovered in 1979. The two O-C residuals are listed in Table 53. The moments of light maxima can be predicted by using the following elements: C = 2444105.697 + 4.236978d*E (38) A very recently obtained radial velocity measurement series (Metzger et al., 1990) gives -15.3 +- 0.3 km/s for the gamma-velocity at an epoch of J.D. 2447002. Further observations, both photometric and spectroscopic, are desirable. Table 53. O-C residuals for V1726 Cyg Norm.max. E O-C W Reference JD2400000+ 44105.697* 0 0.000 3 Platais & Shugarov (1981) 45622.535* 358 0.000 3 Berdnikov (1986) TX Delphini This star is one of the Population II Cepheids in this sample. Because of its belonging to a spectroscopic binary system (Harris and Welch, 1989), TX Del deserves a special attention. The photoelectric observations have been analysed using a new normal light curve, based on Moffett and Barnes' (1984) observations. The O-C graph (see Table 54 and Figure 35) has been approximated with a straight line as follows: C = 2442947.138 + 6.165904d*E (39) +-.012 +-.000023 The deviations from this straight line are in some cases much larger than expected due to the observational errors. The dashed lines in Figure 35 suggest a plausible solution to this anomaly: several successive phase jumps might occur, similarly to the case of Y Oph (Paper IV, p. 42). The pulsation period corresponding to the dashed lines is about 6.1664 days, being in excellent agreement with the period valid before J.D 2436600: P = 6.166585 days (see Paper II, p. 58). Reality of the phase jump approximation, however, has to be proven by future observations. Table 54. O-C residuals for TX Del Norm.max. JD2400000+ E O-C W Reference 35665.108 -1181 -0.097 3 Walraven et al. (1958) 37293.130 -917 0.126 3 Mitchell et al. (1964) 39099.574 -624 -0.040 3 Takase (1969) 40819.978 -345 0.077 3 Pel (1976) 41929.732* -165 -0.032 3 Dean et al. (1977) 42293.514* -106 -0.038 3 Dean et al. (1977) 42361.345* -95 -0.032 3 Harris & Welch (1989) 42947.169 0 0.031 3 Szabados (1980) 43816.622* 141 0.092 3 Harris & Welch (1989) 44359.105* 229 -0.025 3 Moffett & Barnes (1984) 44457.760* 245 -0.024 2 present paper 44926.369* 321 -0.024 3 Moffett & Barnes (1984) 45549.100* 422 -0.049 1 Diethelm (1986) Figure 35. O-C diagram of TX Del Because the orbital period is as short as 133.15 days (Harris and Welch, 1989), the orbital light-time effect cannot cause an observable wave in the O-C diagram. The radial velocity observations of TX Del are not discussed here, because a detailed analysis was performed recently (Harris and Welch, 1989). W Geminorum The reliable O-C residuals from Paper II supplemented with those obtained from the more recent photoelectric observations can be better approximated with a parabola instead of a single sudden period decrease suggested in Paper II. The residuals in Table 55 (see also Figure 36) have been obtained using the equation: C = 2442755.176 + 7.913624d*E (40) +-.024 +-.000047 Table 55. O-C residuals for W Gem Norm.max. E O-C W Reference JD2400000+ 14136.862 -3616 -2.650 1 Pickering (1904) 17318.675 -3214 -2.113 1 Wendell (1913) 17659.022 -3171 -2.052 1 Zeipel (1908) 17944.327 -3135 -1.638 1 van der Bilt (1926b) 18324.126 -3087 -1.693 1 van der Bilt (192¢b) 18807.014 -3026 -1.536 1 van der Bilt (1926b) 19748.851 -2907 -1.420 1 van der Bilt (1926b) 24980.146 -2246 -1.030 1 Carrasco (1932) 25296.648 -2206 -1.073 2 Hellerich (1935) 25716.263 -2153 -0.831 1 Zverev (1936) 26578.991 -2044 -0.738 1 Zverev (1936) 26666.149 -2033 -0.629 1 Kukarkin (1940) 26895.429 -2004 -0.845 1 Kox (1935) 27030.096 -1987 -0.709 1 Florya & Kukarkina (1953) 29364.701 -1692 -0.623 1 Koshkina (1963) 35165.909 -959 -0.102 1 Irwin (1961) 35561.781 -909 -0.089 1 Nikulina (1959) 35569.363 -908 -0.242 2 Walraven et al. (1958) 36186.729 -830 -0.139 1 Latyshev (1969) 37270.925 -693 -0.110 3 Mitchell et al. (1964) 37927.814 -610 -0.051 1 Fridel' (1971) 39043.666 -469 -0.020 3 Wisniewski & Johnson (1963) 39083.150 -464 -0.104 3 Takase (1969) 39502.615 -411 -0.062 3 Wamsteker (1972) 39787.563 -375 -0.004 3 Szabados (1980) 40903.421* -234 0.033 2 Feltz & McNamara (1980) 40927.150 -231 0.021 3 Pel (1976) 41006.204 -221 -0.061 2 Evans (1976) 42755.172 0 -0.004 3 Szabados (1980) 43878.923* 142 0.012 3 Moffett & Barnes (1984) Figure 36. Upper panel: O-C diagram of W Gem Lower panel: gamma-velocities for the same Cepheid Table 56. gamma-velocities of W Gem J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 21600 30 3.0 1.7 4 Sanford (1930) 25468 305 -3.5 0.7 19 Sanford (1930) 41011 29 0.7 0.3 4 Evans (1976) 43445 63 0.9 1.5 8 Wilson et al. (1989) 44015 318 -1.8 0.8 24 Barnes et al. (1987) 46866 1 3.7 0.7 2 Samus (1990) The continuous period decrease is represented with the formula: P = 7.913624d - 3.79d*10^-7*E (41) +-.000047 +-.29 The gamma-velocities determined for W Gem listed in Table 56 are shown plotted in the lower panel of Figure 36. The total variation of the gamma-velocity is larger than 7 km/s, a value far exceeding the uncertainty due to the observational errors. Thus W Gem is a spectroscopic binary Cepheid candidate. The information on the duplicity of W Gem is summarized by Leonard and Turner (1986). They conclude that W Gem does not have an early type companion. The amplitude ratios determined from the BVRI photometry obtained by Moffett and Barnes (1984) imply a late type companion. In any case, W Gem is a promising target for further spectroscopic observations. Similarly to W Gem, the new O-C diagram of this Cepheid is also approximated with a parabola, instead of two linear sections (see Table 57 and the upper panel of Figure 37). The O-C residuals have been obtained using the elements: C = 2442714.955 + 5.529166d*E (42) +-.012 +-.000015 The value of the continuously decreasing period can be calculated as follows: P = 5.529166d - 1.70d*10^-7*E (43) +-.000015 +-.08 The gamma-velocity of RZ Gem shows an obvious variation (see Table 58 and the lower panel of Figure 37). Because this variation seems to be very rapid (i.e. the orbital period is rather short, at least for a Cepheid), a separate gamma-velocity has been determined for each season whenever the radial velocity of RZ Gem was measured. The formal period search routine resulted in the value of 886 days, and this value is considered as the Figure 37. Upper panel: O-C diagram of RZ Gem Lower panel: gamma-velocities for the same Cepheid Table 57. O-C residuals for RZ Gem Norm.max. E O-C W Reference JD2400000+ 18324.181 -4411 -1.623 1 Enebo (1909) 18832.935 -4319 -1.552 1 Enebo (1911) 21277.055 -3877 -1.323 1 Jordan (1929) 26375.422 -2955 -0.847 1 Kukarkin (1940) 28869.451 -2504 -0.472 1 Koshkina (1963) 29383.624 -2411 -0.512 1 Koshkina (1963) 29632.453 -2366 -0.495 1 Chudovicheva (1952) 31556.759 -2018 -0.339 1 Chudovicheva (1952) 32557.503 -1837 -0.374 1 Chudovicheva (1952) 33801.844 -1612 -0.095 1 Koshkina (1963) 34304.930 -1521 -0.164 1 Koshkina (1963) 34327.082 -1517 -0.128 1 Rosino & Nobili (1955) 36831.810 -1064 -0.112 3 Weaver et al. (1960) 37633.563 -919 -0.088 3 Mitchell et al. (1964) 39248.161 -627 -0.007 3 Takase (1969) 42714.927 0 -0.028 3 Szabados (1980) 44440.044* 312 -0.011 3 Moffett & Barnes (1984) 44976.380* 409 -0.004 3 Moffett & Barnes (1984) tentative orbital period. The gamma-velocities folded with this period are plotted in Figure 38. Zero phase is arbitrarily chosen at J.D. 2400000. This orbital radial velocity curve is very promising, but another value for the orbital period cannot be excluded (e.g. 930 days). Duplicity of RZ Gem is supported by other facts, as well. Madore (1977) assumes a B5 photometric companion, and the phase-test by Madore and Fernie (1980) is Figure 38. gamma-velocity values of RZ Gem folded with the 886 day period Table 58. gamma-velocities of RZ Gem J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25175 18 2.1 4.5 2 Joy (1937) 26311 1 11.9 4.5 1 Joy (1937) 26724 1 20.0 4.5 1 Joy (1937) 28058 146 5.0 2.6 4 Joy (1937) 44196 20 13.5 1.8 6 Barnes et al. (1988) 44568 43 11.3 1.3 11 Barnes et al. (1988) 44948 51 10.1 1.8 6 Barnes et al. (1988) 45373 26 14.0 2.9 3 Barnes et al. (1988) 47970 9 20.1 1.8 3 Samus (1990) another piece of evidence for an early type companion. The discrepancy in the gamma-velocity was already apparent in the compilation of Moffett and Barnes (1987). New radial velocity observations would be extremely valuable, especially because the whole orbit can be covered within three years. AD Geminorum The photoelectric O-C residuals including those published in Paper I (and modified according to the new normal light curve) have been approximated with a constant period (see Table 59 and Figure 39): C = 2441694.999 + 3.787990d*E (44) +-.004 +-.000005 Figure 39. Upper panel: O-C diagram of AD Gem Lower panel: gamma-velocities for the same Cepheid Table 59. O-C residuals for AD Gem Norm.max. E O-C W Reference JD2400000+ 36835.010 -1283 0.002 2 Weaver et al. (1960) 37630.445 -1073 -0.041 3 Mitchell et al. (1964) 39202.527 -658 0.025 3 Takase (1969) 41695.016 0 0.017 3 Szabados (1977) 43649.613* 516 0.011 3 Henden (1979) 44407.186* 716 -0.014 3 Moffett & Barnes (1984) 44960.239* 862 -0.007 3 Moffett & Barnes (1984) 44964.034* 863 0.000 3 Connolly et al. (1983) Table 60. gamma-velocities of AD Gem J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 27438 329 35.4 1.7 8 Joy (1937) 44406 195 45.0 0.1 38 Imbert (1983) The gamma-velocity of AD Gem may be variable (see Table 60 and the lower panel of Figure 39). This suspicion can be supported by the facts that Joy's (1937) radial velocity data give a deviating value from the gamma-velocity determined from Imbert's (1983) observations, and Imbert's radial velocity measurements themselves seem to show a systematic trend in the sense that the more recent observations imply a more positive gamma-velocity. Further spectroscopic study of AD Gem would be desirable. DX Geminorum The photographic and photoelectric O-C residuals obtained by fitting the new normal light curve based on Moffett and Barnes' (1984) Figure 40. O-C diagram of DX Gem Table 61. O-C residuals for DX Gem Norm.max. E O-C W Reference JD2400000+ 25295.211 -5283 -0.808 1 Bartkus & Puchinskas (1961) 28594.649 -4231 -1.261 1 Meshkova (1940) 33182.929 -2768 -2.089 1 Satyvaldiev (1970) 35413.074 -2057 -2.194 1 Satyvaldiev (1970) 35554.252 -2012 -2.171 1 Walraven et al. (1958) 35896.275 -1903 -2.057 1 Bartkus & Puchinskas (1961) 36275.769 -1782 -2.113 1 Bartkus & Puchinskas (1961) 40793.694 -342 -1.150 3 Pel (1976) 41866.721 0 -0.901 3 Szabados (1977) 43165.632 414 -0.617 2 Szabados (1977) 43614.804* 557 -0.004 1 Henden (1979) 44439.787* 820 0.006 2 Moffett & Barnes (1984) 44954.210* 984 -0.003 3 Moffett & Barnes (1984) observations are listed in Table 61 (see also Figure 40). The following ephemeris was used when computing the O-C residuals: C = 2441867.622 + 3.136779d*E (45) +-.012 +-.000013 The photoelectric data clearly show a recent phase jump with an amplitude of 0.2 period. This phase jump can also be suspected in the O-C diagram based on the Sonneberg photographic observations (Hacke, 1989). Although its scatter is very wide, Hacke's O-C diagram also shows a further period change (after J.D. 2445000) that cannot be seen here, because no such recent photoelectric observations are available. Consequently the ephemeris given in Eq. (45) might not be correct for predicting the maxima occurring even in the near future (the true period is expected to be shorter: 3.13669 days according to Hacke). Another phase jump might occur just before J.D. 2440500, but this phase shift cannot be traced with the photoelectric observation. The following values of the pulsation period can be derived from the existing data: before J.D. 2436500 P = 3.136387 +- 0.000041 days between J.D. 2440700 and 2443200 P = 3.137486 +- 0.000005 days after J.D. 2443600 P = 3.136779 +- 0.000013 days. The occurrence of the phase jump(s) in the O-C diagram of DX Gem is in accordance with the suspected spectroscopic binary nature of this Cepheid (Burki, 1985). Unfortunately the radial velocity data have not been published yet. Figure 41. Upper panel: O-C diagram of Zeta Gem Lower panel: gamma-velocities for the same Cepheid Table 62. O-C residuals for Zeta Gem Norm.max. E O-C W Reference JD2300000+ 96027.855 -4704 -11.635 1 Hagen (1903) 96088.465 -4698 -11.925 1 Argelander (1869) 96474.176 -4660 -11.917 1 Argelander (1869) 96565.026 -4651 -12.418 1 Hagen (1903) 97154.525 -4593 -11.623 1 Argelander (1869) 97459.537 -4563 -11.113 1 Hagen (1903) 97519.975 -4557 -11.576 1 Argelander (1869) 97875.456 -4522 -11.347 1 Argelander (1869) 98241.363 -4486 -10.843 1 Argelander (1869) 98536.305 -4457 -10.253 1 Hagen (1903) 98586.501 -4452 -10.807 1 Argelander (1869) 98982.738 -4413 -10.423 1 Argelander (1869) 99317.451 -4380 -10.663 1 Argelander (1869) 99632.685 -4349 -10.081 1 Zinner & Wachmann (1931) 99723.768 -4340 -10.349 1 Argelander (1869) 2400000+ 00099.937 -4303 -9.733 1 Argelander (1869) 01927.501 -4123 -9.182 1 Argelander (1869) 02313.385 -4085 -9.001 1 Valentiner (1900) 02648.656 -4052 -8.682 1 Valentiner (1900) 02993.591 -4018 -8.850 1 Valentiner (1900) 03785.426 -3940 -8.720 1 Valentiner (1900) 03786.543 -3940 -7.603 1 Zinner & Wachmann (1931) 04090.631 -3910 -8.018 1 Valentiner (1900) 04466.252 -3873 -7.949 1 Valentiner (1900) 04608.424 -3859 -7.878 1 Zinner & Wachmann (1931) 04831.417 -3837 -8.187 1 Valentiner (1900) 05166.882 -3804 -7.674 1 Valentiner (1900) 05552.714 -3766 -7.545 1 Valentiner (1900) 05928.082 -3729 -7.730 1 Valentiner (1900) 10009.990 -3327 -6.152 1 Hagen (1891) 13564.137 -2977 -4.531 1 Plassmann (1900) 13950.417 -2939 -3.954 1 Plassmann (1900) 13979.885 -2936 -4.936 1 Pickering (1904) 14335.691 -2901 -4.382 1 Pickering (1904) 14478.219 -2887 -3.955 1 Plassmann (1900) 14487.821 -2886 -4.503 1 Wirtz (1901) 15320.879 -2804 -3.752 1 Plassmann (1900,1901,1908) 15605.224 -2776 -3.609 1 Tass (1925) 15787.268 -2758 -4.266 1 Plassmann (1908) 15787.949 -2758 -3.585 1 Nijland (1923) 15797.704 -2757 -3.980 1 Kopff (1902) 15970.085 -2740 -4.150 1 van der Bilt (1926a) 16193.288 -2723 -3.498 1 Plassmann (1908) 16172.918 -2720 -4.319 1 Lau (1909) 16406.864 -2697 -3.824 1 Gotz (1906) 16508.718 -2687 -3.471 1 Plassmann (1908) 16711.866 -2667 -3.325 1 Olivier (1952) 16721.611 -2666 -3.730 1 van der Bilt (1926a) 16874.025 -2651 -3.567 1 Tass (1925) 16874.320 -2651 -3.272 1 Tass (1925) 16883.963 -2650 -3.779 1 Plassmann (1908) 16894.896 -2649 -2.996 1 Schiller (1906) 17228.908 -2616 -3.936 1 van der Bilt (1926a) 17238.785 -2615 -4.209 1 Plassmann (1908) Table 62. (cont.) Norm.max. E O-C W Reference JD2400000+ 17310.450 -2608 -3.595 1 Lohnert (1909) 17594.592 -2580 -3.655 1 Olivier (1952) 17604.986 -2579 -3.411 1 Nijland (1923) 17614.853 -2578 -3.694 1 van der Bilt (1926a) 17777.885 -2562 -3.063 1 Plassmann (1908) 17939.841 -2546 -3.509 1 Nijland (1923) 17950.165 -2545 -3.335 1 van der Bilt (1926a) 18315.716 -2509 -3.186 1 Nijland (1923) 18417.041 -2499 -3.362 1 Mundler (1911) 18559.427 -2485 -3.077 1 Olivier (1952) 18640.796 -2477 -2.909 1 Nijland (1923) 19899.406 -2353 -2.908 1 Kaiser (1915) 20904.713 -2254 -2.458 1 Luyten (1922) 21320.897 -2213 -2.427 1 Luyten (1922) 22244.104 -2122 -2.877 1 Rabe (1923) 22366.187 -2110 -2.595 3 Guthnick (1921) 22375.973 -2109 -2.959 1 Bellemin (1922) 22721.639 -2075 -2.395 1 Leiner (1922) 22731.566 -2074 -2.619 1 Rabe (1923) 22732.368 -2074 -1.817 1 Gallisot (1923) 22803.383 -2067 -1.852 1 Bellemin (1922) 23056.382 -2042 -2.605 1 Rabe (1923) 23137.772 -2034 -2.415 1 Nielsen (1927a) 23168.397 -2031 -2.241 1 Zverev (1936) 23269.793 -2021 -2.345 3 Bottlinger (1928) 23442.428 -2004 -2.262 1 Leiner (1928) 23543.967 -1994 -2.223 1 Parenago (1938) 23736.995 -1975 -2.047 1 Hopmann (1926) 23797.392 -1969 -2.550 1 Leiner (1928) 24203.841 -1929 -2.104 1 Leiner (1928) 24711.413 -1879 -2.036 1 Leiner (1928) 24761.751 -1874 -2.448 1 Kukarkin (1940) 25228.769 -1828 -2.334 1 Hellerich (1935) 25280.295 -1823 -1.558 1 Leiner (1928) 25310.362 -1820 -1.941 1 Collmann (1930) 25340.804 -1817 -1.950 3 Gussow (1930) 25462.360 -1805 -2.194 1 Kukarkin (1940) 25625.169 -1789 -1.787 1 Zverev (1936) 26325.312 -1720 -1.999 1 Zverev (1936) 26802.918 -1673 -1.446 3 Hall (1934) 27056.638 -1648 -1.478 1 Florya & Kukarkina (1953) 27198.938 -1634 -1.279 1 Nielsen (1941) 28436.956 -1512 -1.570 1 Gunther (1939) 32883.249 -1074 -1.010 3 Eggen (1951) 34416.368 -923 -0.552 3 Harris (1953) 36639.347 -704 -0.439 1 Azarnova (1960a) 37004.583 -668 -0.606 1 Mayall (1964) 37258.567 -643 -0.373 1 Mitchell et al. (1964) 37979.154 -572 -0.442 2 Williams (1966) 38527.582 -518 -0.118 3 Wisniewski & Johnson (1968) 39420.773 -430 -0.133 3 Takase (1969) 39765.870 -396 -0.139 2 Sudzius (1969) 39796.556 -393 0.097 3 Szabados (1981) 40892.664* -285 -0,003 2 Feltz & McNamara (1980) Table 62. (cont.) Norm.max. E O-C W Reference JD2400000+ 41004.565 -274 0.247 2 Evans (1976) 42354.510 -141 0.232 1 Scarfe (1976) 42465.897 -130 -0.031 3 Depenchuk (1980) 43552.060* -23 0.074 3 Moffett & Barnes (1984) 43785.473 0 0.035 3 Szabados (1981) 43805.927 2 0.189 2 Depenchuk (1980) 44039.297* 25 0.107 3 Moffett & Barnes (1984) 44140.727* 35 0.036 1 Schmidt & Parsons (1982) 44709.348* 91 0.253 1 Ridgway et al. (1982) Figure 42. Delta(O-C) diagram of Zeta Gem The old visual observations have also been taken into account during the re-discussion of the O-C diagram, because the previous study (see Paper III) revealed some shorter time-scale deviations superimposed on the general parabolic trend of the O-C diagram. In order to obtain the value of the period decrease as accurately as possible, a long time-base data set is necessary. The currently used ephemeris is deduced from the best parabolic fit (see Table 62 and Figure 41): C = 2443785.438 + 10.150074d*E (46) +-.053 +-.000053 while the continuous period decrease is as follows: P = 10.150074d - 10.76d*10^-7*E (47) +-.000053 +-.23 The deviations from the least squares fitted parabola are shown on the Delta(O-C) diagram in Figure 42. Here only the photoelectric and the photographic O-C residuals have been taken into account. The trend of the deviations resembles the phase jump appearing in the O-C diagram of a number of Cepheids. In most cases, however, the O-C diagrams are linear, and Zeta Gem would be the first case when a phase jump occurs on a parabolic O-C diagram. It cannot be excluded that this phase shift is not the commonly appearing one in binary Cepheids, but its origin can be explained Table 63. gamma-velocities of Zeta Gem J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 14419 170 13.5 0.8 15 Henroteau (1925) 14650 39 6.0 2.1 3 Campbell (1899) 14851 158 7.1 0.5 44 Henroteau (1925) 19799 1312 11.3 2.1 3 Spencer Jones (1928) 21014 417 7.4 0.5 42 Hase (1929) 21592 1 12.2 2.0 1 Abt (1970) 22606 188 5.1 0.6 23 Hase (1929) 23541 80 6.3 0.7 22 Jacobsen (1926) 23837 21 6.9 0.5 43 Henroteau (1925) 24664 496 5.6 0.6 25 Hase (1929) 27413 86 1.6 2.0 2 Abt (1970) 29028 393 6.5 0.9 6 Abt (1970) 30368 408 7.1 0.1 13 Scarfe (1976) 40506 636 7.8 0.1 15 Abt & Levy (1974) 40625 224 6.5 0.1 12 Scarfe (1976) 40990 21 6.2 0.3 6 Evans (1976) 43463 57 4.4 1.2 12 Wilson et al. (1989) 43619 135 5.7 0.5 4 Beavers & Eitter (1986) 43934 218 5.0 0.8 21 Barnes et al. (1987) 44313 540 4.8 0.4 19 Jacobsen & Wallerstein (1982) 44916 8 5.4 0.3 7 Beavers & Eitter (1986) 46951 296 6.7 0.4 10 Samus (1990) in terms of the stellar activity (see Hall, 1990, and the general remarks in this paper, on page 233). The gamma-velocities determined from the available radial velocity data (see Table 63 and the lower panel of Figure 41) do not show a clear variation but changes up to 4-5 km/s cannot be excluded. Even in this latter case, no observable light-time effect is expected in the O-C diagram. A long series of homogeneous radial velocity observations will hopefully solve the problem of duplicity of Zeta Gem. V Lacertae The photographic and photoelectric O-C residuals listed in Paper I have been supplemented with the more recent photoelectric data. The O-C diagram continues to be parabolic (see Figure 43). During the present analysis the O-C residuals (listed in Table 64) have been calculated using the elements: C = 2441907.706 + 4.983179d*E (48) +-.006 +-.000003 The continuous decrease in the period can be characterized as follows: P = 4.983179d - 1.09d*10^-7*E (49) +-.000003 +-.02 Figure 43. Upper panel: O-C diagram of V Lac Lower panel: gamma-velocities for the same Cepheid Table 64. O-C residuals for V Lac Norm.max. E O-C W Reference JD2400000+ 19716.574 -4453 -1.036 1 Martin & Plummer (1916) 20070.390 -4382 -1.026 2 Martin & Plummer (1916) 20633.512 -4269 -1.003 2 Martin & Plummer (1916) 20817.920 -4232 -0.972 2 Hertzsprung (1922) 21844.515 -4026 -0.912 2 Jordan (1929) 26558.980 -3080 -0.535 1 Zonn (1933) 27092.226 -2973 -0.489 1 Zonn (1933) 33635.445 -1660 -0.184 1 Solov'yov (1952) 35788.311* -1228 -0.051 3 Bahner & Mavridis (1977) 36266.691* -1132 -0.056 3 Bahner & Mavridis (1977) 36794.894 -1026 -0.070 3 Weaver et al. (1960) 36809.829 -1023 -0.085 2 Oosterhoff (1960) 36834.777 -1018 -0.053 2 Bahner et al. (1962) 37348.104 -915 0.007 2 Mitchell et al. (1964) 37422.781 -900 -0.064 1 Golovatyj (1964) 38070.607 -770 -0.051 1 Golovatyj (1964) 39754.969 -432 -0.004 2 Szabados (1977) 41907.688 0 -0.018 3 Szabados (1977) 43711.644* 362 0.027 1 Henden (1979) 44329.517* 486 -0.014 3 Moffett & Barnes (1984) 44957.385* 612 -0.027 3 Moffett & Barnes (1984) Because traces of a wave-like distorsion are apparently superimposed on the fitted parabola, light-time effect was also searched for. Two possible periods could be deduced in this way: 4020 and 8040 days. It is, however, improbable that either of these values corresponds to the orbital period Table 65. gamma-velocities of V Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25632 440 -20.4 1.6 9 Joy (1937) 44771 427 -26.0 1.3 11 Barnes et al. (1987) 47970 164 -25.7 0.7 8 Samus (1990) because the waves caused by these periods are not compatible with the gamma-velocity changes to be discussed below. The gamma-velocities derived from the available radial velocity data are listed in Table 65 (see also the lower panel of Figure 43). The discrepancy noted by Moffett and Barnes (1987) is clearly seen between Joy's (1937) and the more recent data. Nevertheless, additional radial velocity measurements are needed to confirm the spectroscopic binary nature of V Lac. As a matter of fact, Oosterhoff (1960) assumed a blue photometric companion, but the spectrophotometric study performed by Miller and Preston (1964a) does not support the existence of a blue secondary star. X Lacertae The recent photoelectric data imply that the O-C diagram of X Lac can be properly interpreted in terms of phase jumps (see Figure 44). A new Figure 44. Upper panel: O-C diagram of X Lac Lower panel: gamma-velocities for the same Cepheid Table 66. O-C residuals for X Lac Norm.max. E O-C W Reference JD2400000+ 17551.212 -4626 -1.518 1 Seares (1906,1907) 19870.408 -4200 -1.604 1 Martin & Plummer (1916) 20012.081 -4174 -1.483 1 Hertzsprung (1922) 20741.601 -4040 -1.502 1 Martin & Plummer (1916) 21460.459 -3908 -1.295 1 Hertzsprung (1922) 21487.423 -3903 -1.552 1 Jordan (1929) 23458.068 -3541 -1.752 1 Doberck (1924) 26539.666 -2975 -1.640 1 Kukarkin (1940) 26550.469 -2973 -1.726 1 Zonn (1933) 27105.928 -2871 -1.598 1 Zonn (1933) 33536.037 -1690 -1.223 1 Solov'yov (1952) 34358.247 -1539 -1.105 1 Romano (1955) 35757.696 -1282 -0.847 3 Bahner & Mavridis (1971) 36122.521 -1215 -0.792 1 Makarenko (1969) 36193.241 -1202 -0.848 3 Bahner & Mavridis (1971) 36487.298 -1148 -0.784 1 Makarenko (1969) 36786.745 -1093 -0.775 3 Weaver et al. (1960) 36797.603 -1091 -0.806 3 Oosterhoff (1960) 36835.657 -1084 -0.862 3 Bahner et al. (1962) 36841.091 -1083 -0.872 1 Makarenko (1969) 37200.493 -1017 -0.796 1 Makarenko (1969) 37216.790 -1014 -0.831 3 Mitchell et al. (1964) 37576.258 -948 -0.689 1 Makarenko (1969) 37635.956 -937 -0.878 1 Golovatyj (1964) 37946.397 -880 -0.764 1 Makarenko (1969) 38305.854 -814 -0.632 1 Makarenko (1969) 38866.704 -711 -0.754 1 Makarenko (1969) 39743.265 -550 -0.522 3 Szabados (1980) 40195.303 -467 -0.363 3 Asteriadis et al. (1977) 41333.418* -258 -0.111 1 Feltz & McNamara (1980) 42738.184 0 0.020 3 Szabados (1980) 43696.330 176 -0.034 2 Henden (1979) 44327.880* 292 -0.026 3 Moffett & Barnes (1984) 44948.587* 406 0.028 3 Moffett & Barnes (1984) normal light curve was used during this study, and the O-C residuals listed in Table 66 have been calculated according to the ephemeris: C = 2442738.164 + 5.444322d*E (50) +-.015 +-.000057 The three intervals during which the pulsation period was nearly the same are listed below: between J.D. 2417550 and 2427100 P = 5.444212 +- 0.000066 days between J.D. 2435750 and 2437200 P = 5.444403 +- 0.000080 days after J.D. 2442700 P = 5.444322 +- 0.000057 days. The pulsation period valid during the intermediate intervals cannot be determined reliably. The value of the phase jump was about 0.18 day (=0.15 cycle) in both cases. The variation in the gamma-velocity can be suspected on the basis of the radial velocity data (see Table 67 and the lower panel of Figure 44). Table 67. gamma-velocities of X Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 24959 201 -26.2 1.5 10 Joy (1937) 44061 1 -32.6 4.0 1 Barnes et al. (1988) 44608 175 -27.1 1.5 8 Barnes et al. (1988) 45340 2 -28.9 4.0 2 Barnes et al. (1988) Moffett and Barnes (1987) also mentioned this discrepancy. There are, however, other pieces of evidence for the binary nature of X Lac: the method of Madore and Fernie (1980) indicated a blue photometric companion, while Usenko (1990b) assumes a B7 companion from the two-colour diagram. In addition, the phase jumps seen in the O-C diagram give an independent evidence for duplicity of X Lacertae. Y Lacertae The new photoelectric O-C residuals confirm the earlier conclusion about the constancy of the pulsation period of Y Lac. The new ephemeris used for computing the O-C residuals listed in Table 68 is as follows: C = 2441746.722 + 4.323769d*E (51) +-.004 +-.000003 The plot of the residuals in Figure 45, however, shows hints of a wave-like pattern with a cycle-length of more than ten thousand days. Obviously, more observations are needed to decide whether these deviations from the straight line can be attributed to the light-time effect or not. Figure 45. Upper panel: O-C diagram of Y Lac Lower panel: gamma-velocities for the same Cepheid Table 68. O-C residuals for Y Lac Norm.max. E O-C W Reference JD2400000+ 21658.488 -4646 -0.003 1 Jordan (1929) 21818.386 -4609 -0.085 1 Martin & Plummer (1919) 33125.147 -1994 0.020 3 Eggen (1951) 33609.468 -1882 0.079 1 Solov'yov (1952) 35710.744* -1396 0.004 3 Bahner & Mavridis (1977) 36112.859* -1303 0.008 3 Bahner & Mavridis (1977) 36834.936 -1136 0.016 3 Bahner et al. (1962) 37366.745 -1013 0.001 3 Mitchell et al. (1964) 41431.062* -73 -0.025 3 Feltz & McNamara (1980) 41746.720 0 -0.002 3 Szabados (1977) 43039.515* 299 -0.014 3 Chekhanikhina (1982) 43683.766* 448 -0.005 3 Henden (1979) 44349.632* 602 0.001 3 Moffett & Barnes (1984) 44457.722* 627 -0.003 1 present paper 44924.690* 735 -0.002 3 Moffett & Barnes (1984) Table 69. gamma-velocities of Y Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25943 564 -19.8 1.5 10 Joy (1937) 44576 225 -22.7 0.9 21 Barnes et al. (1988) The two gamma-velocities determined from the available radial velocity data (see Table 69) deviate from each other. This discrepancy has been already noted by Moffett and Barnes (1987). An A0V companion has been discovered in an IUE spectrum (Evans et al., 1990a), thus confirming Madore's (1977) earlier suggestion concerning the existence of a blue companion to Y Lac. The spectroscopic binary nature of this Cepheid, however, has to be studied further. The recent O-C residuals confirm the parabolic shape of the O-C diagram (see Table 70 and Figure 46). The ephemeris is slightly modified as compared with that determined in Paper III. When constructing the present O-C diagram, the following ephemeris was used: C = 2442827.136 + 10.885642d*E (52) +-.007 +-.000021 The decrease of the pulsation period is as follows: P = 10.885642d - 6.4d*10^-8*E (53) +-.000021 +-2.8 Because Z Lac is a member in a newly discovered spectroscopic binary (Moffett and Barnes, 1987; Gieren, 1989a) an attempt was made to search for a light-time effect in the O-C diagram. The only reasonable fit Figure 46. Upper panel: O-C diagram of Z Lac Lower panel: gamma-velocities for the same Cepheid Table 70. O-C residuals for Z Lac Norm.max. E O-C W Reference JD2400000+ 20108.601 -2087 -0.200 1 Hertzsprung (1922) 21534.721 -1956 -0.099 1 Hertzsprung (1922) 26542.105 -1496 -0.111 1 Zonn (1933) 27108.157 -1444 -0.112 1 Zonn (1933) 28229.624 -1341 0.134 1 Gur'yev (1937) 28741.160 -1294 0.045 1 Gur'yev (1938) 33084.478 -895 -0.008 3 Eggen (1951) 33519.846 -855 -0.066 1 Solov'yov (1952) 34575.782 -758 -0.037 1 Chuprina (1954a) 34967.738 -722 0.036 1 Chuprina (1956) 35751.452 -650 -0.017 3 Bahner & Mavridis (1971) 36230.385 -606 -0.052 3 Bahner & Mavridis (1971) 36829.095 -551 -0.052 3 Bahner et al. (1962) 37199.191 -517 -0.068 3 Mitchell et al. (1964) 37928.575 -450 -0.022 1 Girnyak (1964) 39180.423 -335 -0.023 3 Takase (1969) 39768.312 -281 0.041 2 Szabados (1981) 40083.936 -252 -0.018 3 Asteriadis et al. (1977) 40791.548 -187 0.027 3 Asteriadis et al. (1977) 41324.953* -138 0.036 3 Feltz & McNamara (1980) 42674.769* -14 0.032 3 Szabados (1977) 43708.881* 81 0.008 3 Szabados (1977) 44318.486* 137 0.017 3 Moffett & Barnes (1984) 44938.988* 194 0.037 3 Moffett & Barnes (1984) 45679.155* 262 -0.019 3 Berdnikov (1986) 46702.305* 356 -0.120 1 "Carlsberg" (1989) 47126.952* 395 -0.013 3 Gieren (1989a) results in a period of about, 8700 days but a period as long as this can hardly correspond to the orbital period of the system because the variation in the gamma-velocity suggests a much shorter value. Table 71. gamma-velocities of Z Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 22936 15 -29.1 3.2 3 Joy (1937) 24613 277 -18.7 4.5 2 Joy (1937) 25665 282 -26.0 2.0 6 Joy (1937) 44131 95 -31.7 4.0 2 Barnes et al. (1988) 44613 188 -35.0 1.6 7 Barnes et al. (1988) 45343 1 -40.4 2.8 3 Barnes et al. (1988) 47134 7 -39.9 0.3 7 Gieren (1989a) 47373 1 -16.4 0.5 1 Samus (1990) 47472 1 -34.5 0.7 1 Samus (1990) The gamma-velocities of Z Lac are listed in Table 71. As is clearly seen from the lower panel of Figure 46, the orbital period is much shorter than 8700 days, perhaps it can be as short as one year. Further speculation on the value of the orbital period is untimely yet. RR Lacertae A new normal light curve based on Moffett and Barnes' (1984) observations was used when studying the period changes of RR Lac. As a consequence, a systematic correction of -0.026 day has been applied to the normal maxima taken from Paper II. The new values of the O-C residuals (see Table 72) have been computed with the formula: C = 2442776.681 + 6.416289d*E (54) +-.006 +-.000011 The best representation of the O-C plot is the approximation with a Table 72. O-C residuals for RR Lac Norm.max. E O-C W Reference JD2400000+ 20005.526 -3549 0.255 2 Hertzsprung (1922) 21429.895 -3327 0.208 2 Hertzsprung (1922) 21667.339 -3290 0.249 1 Jordan (1929) 26537.195 -2531 0.141 1 Zonn (1933) 27095.442 -2444 0.171 1 Zonn (1933) 32998.324 -1524 0.067 3 Eggen (1951) 33537.290 -1440 0.065 1 Solov'yov (1952) 34968.107 -1217 0.050 1 Azarnova (1957) 36033.188 -1051 0.027 3 Bahner & Mavridis (1971) 36835.212 -926 0.015 3 Bahner et al. (1962) 37213.735 -867 -0.023 3 Mitchell et al. (1964) 37579.554 -810 0.067 1 Girnyak (1964) 38240.312 -707 -0.053 1 Girnyak (1964) 40582.324 -342 0.014 3 Asteriadis et al. (1977) 42776.694 0 0.013 3 Szabados (1980) 44335.840* 243 0.001 3 Moffett & Barnes (1984) 44932.557* 336 0.003 3 Moffett & Barnes (1984) Figure 47. Upper panel: O-C diagram of RR Lac Lower panel: gamma-velocities for the same Cepheid Table 73. gamma-velocities of RR Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25244 526 -31.9 2.6 4 Joy (1937) 26502 255 -37.9 2.0 6 Joy (1937) 44510 288 -39.8 1.5 8 Barnes et al. (1987) parabola (see Figure 47) indicating a continuously increasing pulsation period: P = 6.416289d + 4.1*10^-8*E (55) +-.000011 +-0.6 There are three values of the gamma-velocity that can be deduced from the published radial velocity measurements (see Table 73). These data indicate that the gamma-velocity of RR Lac possibly varies, in accordance with the study performed by Moffett and Barnes (1987) who also noticed the discordant gamma-velocities. Further radial velocity observations would be very important. BG Lacertae BG Lac keeps its constant pulsation period. The new value based on solely photoelectric data (see Table 74) only slightly differs from that determined in Paper II. The O-C residuals plotted in Figure 48 have been approximated with the line: C = 2442673.187 + 5.331902d*E (56) +-.009 +-.000010 Figure 48. Upper panel: O-C diagram of BG Lac Lower panel: gamma-velocities for the same Cepheid Table 74. O-C residuals for BG Lac Norm.max. E O-C W Reference JD2400000+ 33129.068 -1790 -0.014 3 Eqgen (1951) 35938.962 -1263 -0.033 3 Bahner & Mavridis (1977) 36834.834 -1095 -0.080 2 Bahner et al. (1962) 37261.307 -1015 0.001 3 Mitchell et al. (1964) 39772.614 -544 -0.018 3 Szabados (1980) 41361.567* -246 0.028 1 Feltz & McNamara (1980) 42673.231 0 0.044 3 Szabados (1980) 44400.699* 324 -0.024 3 Moffett & Barnes (1984) 44896.574* 417 -0.016 3 Moffett & Barnes (1984) Table 75. gamma-velocities of BG Lac J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 27048 180 -14.5 2.0 6 Joy (1937) 27868 195 -23.1 3.2 3 Joy (1937) 44640 309 -18.0 0.9 23 Barnes et al. (1988) As to the duplicity of BG Lac, Madore (1977) assumed a B8 type photometric companion. No blue companion star was, however, found during the IUE-study of this Cepheid (Bohm-Vitense and Proffitt, 1985). Nevertheless, the gamma-velocity seems to be variable judging from the data listed in Table 75. Further radial velocity measurements are necessary to point out the possible orbital effect more clearly. T Monocerotis The O-C diagram of T Mon published in Paper III clearly showed a continuous period increase during an interval of more than one century. A Figure 49. Upper panel: O-C diagram of T Mon Lower panel: gamma-velocities for the same Cepheid Table 76. O-C residuals for T Mon Norm.max. E O-C W Reference JD2400000+ 25302.270 -684 0.067 Hellerich (1935) 32840.908 -405 0.098 2 Eggen (1951) 35191.449 -318 -0.109 2 Irwin (1961) 35488.134 -307 -0.645 2 Walraven et al. (1958) 37056.133 -249 0.188 2 Mitchell et al. (1964) 37974.560 -215 -0.068 1 Williams (1966) 38758.533 -186 0.322 3 Wisniewski & Johnson (1968) 39352.984 -164 0.331 1 Takase (1969) 39812.386 -147 0.391 3 Szabados (1981) 40892.759* -107 -0.040 2 Feltz & McNamara (1980) 40919.687 -106 -0.132 3 Pel (1976) 41001.207 -103 0.328 1 Evans (1976) 41730.626 -76 0.204 3 Landis (1976) 41757.170* -75 -0.272 3 Dean et al. (1977) 42567.909* -45 -0.136 3 Dean et al. (1977) 43513.475* -10 -0.273 3 Dean (1981) 43784.184 0 0.235 3 Szabados (1981) 43810.773* 1 -0.196 3 Moffett & Barnes (1984) 44081.479* 11 0.309 3 Eggen (1983b) 44702.532* 34 -0.100 3 Coulson & Caldwell (1985) 45702.336* 71 -0.040 3 Berdnikov (1986) closer look at the O-C residuals revealed deviations from the uniform period decrease (see Figure 53 in Paper III). These deviations are much too large to be explained by the light-time effect. Because the photoelectric O-C residuals fall on a branch of such systematic deviation, Table 77. gamma-velocities of T Mon J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 17174 21 22.1 2.1 3 Frost (1906) 21580 30 26.2 2.0 5 Sanford (1927) 23262 268 30.0 3.0 2 Harper (1934) 23698 181 30.1 0.8 27 Sanford (1927) 23934 195 28.4 1.1 15 Sanford (1927) 32327 575 29.8 0.5 20 Sanford (1956) 40682 364 28.7 0.3 25 Wallerstein (1972) 40984 11 29.2 1.1 3 Schmidt (1974) 41006 15 28.1 0.3 4 Evans (1976) 41285 8 29.6 0.2 8 Coulson (1983) 41713 52 28.6 0.2 14 Coulson (1983) 43204 1 28.0 0.6 1 Coulson (1983) 43435 60 20.2 1.6 7 Wilson et al. (1989) 44023 314 25.0 0.8 25 Barnes et al. (1987) 44266 61 21.0 0.6 27 Coulson (1983) 44723 150 21.4 0.5 31 Coulson (1983) 46862 7 18.7 0.6 3 Samus (1990) 47134 7 17.0 0.2 13 Gieren (1989b) the O-C diagram covering the last forty years cannot be approximated with a parabola. Instead, a constant period has been assumed here, when constructing the O-C diagram from the data points listed in Table 76 (and shown plotted in Figure 49): C = 2443783.949 + 27.020097d*E (57) +-.048 +-.000305 Note that this ephemeris is not valid before J.D. 2425000, and the reality of predicting future maxima using Eq. (57) may be doubted. The duplicity of T Mon was frequently discussed in the last decade. In the most recent detailed study Gieren (1989b) concludes that the orbital period is about 175 years. The gamma-velocities have been redetermined in the present study, supplemented with the results obtained from the recently published radial velocity measurements (see Table 77 and the lower panel of Figure 49). The new values of the gamma-velocity also confirm Gieren's conclusion concerning the long orbital period. Although the orbital motion of this Cepheid causes a light-time effect with a full amplitude exceeding 0.1 day, the effect cannot be pointed out in the O-C diagram because of the wide scatter due to the long pulsation period of T Mon. Being a possible member in the Mon OB2 association (Gieren, 1988), T Mon can be an important calibrating Cepheid. SV Monocerotis The new value of the pulsation period determined only from photoelectric data (see Table 78 and Figure 50) is somewhat shorter than that determined in Paper III. The O-C residuals have been calculated using the elements: C = 2443794.249 + 15.232582d*E +-.019 +-.000073 Figure 50. Upper panel: O-C diagram of SV Mon Lower panel: gamma-velocities for the same Cepheid Table 78. O-C residuals for SV Mon Norm.max. E O-C W Reference JD2400000+ 35477.219 -546 -0.040 3 Walraven et al. (1958) 37564.109 -409 -0.014 3 Mitchell et al. (1964) 37899.291 -387 0.051 2 Eggen (1969) 39209.191 -301 -0.051 2 Wamsteker (1972) 39346.203 -292 -0.132 2 Takase (1969) 40732.705 -201 0.205 3 Pel (1976) 42865.108 -61 0.047 2 Dean (1977) 43489.586* -20 -0.011 1 Dean (1981) 43794.342 0 0.093 3 Szabados (1981) 44449.175* 43 -0.075 2 Eggen (1983b) 44525.339* 48 -0.074 3 Coulson & Caldwell (1985) 44890.941* 72 -0.054 3 Moffett & Barnes (1984) 44967.090* 77 -0.068 3 Coulson & Caldwell (1985) 45683.183* 124 0.094 2 Berdnikov (1986) Table 79. gamma-velocities of SV Mon J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25581 625 27.3 1.5 10 Joy (1937) 44264 64 24.9 0.2 26 Coulson & Caldwell (1985) 44705 370 24.2 0.9 19 Barnes et al. (1988) 44723 150 26.5 0.2 31 Coulson & Caldwell (1985) There is no evidence for duplicity of SV Mon published in the literature, and correspondingly, the gamma-velocities listed in Table 79 show no sign of variability. CV Monocerotis Considerable attention has been paid to this not very bright Cepheid, because it is a suspected member of an anonymous open cluster (see Walker, 1987 and the references therein). Turner's (1978) photometric data were used for determining a new normal light curve. The earlier (partly photographic) observations have been analysed again because of the existence of the phase jump pointed out in Paper II. The O-C residuals listed in Table 80 have been obtained using the ephemeris: C = 2442773.064 + 5.378804d*E (59) +-.013 +-.000026 The O-C diagram in Figure 51 confirms the occurrence of the phase jump at about J.D. 2436000. The shift between the two almost parallel sections is about -0.34 day. Before J.D. 2436000 the pulsation period was 5.378757 +- 0.000011 days, i.e. practically the same value as given in the current ephemeris (Eq. 59). Unfortunately the observational efforts have not been extended towards the spectroscopy of CV Mon. Only one set of radial velocity data is available in the literature (Barnes et al., 1988). A second epoch radial velocity observation would be extremely valuable for pointing out the spectroscopic binary nature of this Cepheid. The companion is expected to be a B7 star (Madore, 1977), or at least a blue one (Pel, 1978). Table 80. O-C residuals for CV Mon Norm.max. E O-C W Reference JD2400000+ 29805.169 -2411 0.401 1 Teplitskaya (1951) 30778.710 -2230 0.379 1 Teplitskaya (1951) 31908.250 -2020 0.371 1 Filatov (1961) 35447.480 -1362 0.347 1 Filatov (1961) 36571.216 -1153 -0.087 2 Arp (1960) 41035.782 -323 0.072 3 Pel (1976) 42773.133 0 0.069 3 Szabados (1980) 42842.984 13 -0.004 3 Turner (1978) 43886.453* 207 -0.023 2 Dean (1981) 44499.642* 321 -0.018 2 Moffett & Barnes (1984) 44951.475* 405 -0.005 3 Moffett & Barnes (1984) 45962.734* 593 0.039 1 Visvanathan (1989) 46468.211* 687 -0.091 2 present paper Figure 51. O-C diagram of CV Mon V465 Monocerotis There are only four photoelectric maxima determined for V465 Mon (see Table 81 and Figure 52). The weighted least squares fit resulted in the ephemeris: C = 2441698.707 + 2.713006d*E (60) +-.028 +-.000048 The behaviour of the pulsation period prior to J.D. 2441000 is poorly known. As is seen in the previous O-C diagram (in Paper I), the period has not been constant since the discovery of light variability. V465 Mon deserves more attention. Further photometric data will hopefully explain and remove the zero point difference between the magnitude scales used by the three observers involved in Table 81. Figure 52. O-C diagram of V465 Mon Table 81. O-C residuals for V465 Mon Norm.max. E O-C W Reference JD2400000+ 41698.687 0 -0.020 3 Szabados (1977) 43166.515 541 0.072 2 Szabados (1977) 43676.484* 729 -0.004 3 Henden (1979) 44235.292* 935 -0.076 1 Diethelm & Tammann (1982) According to Burki (1985) V465 Mon is a component in a spectroscopic binary system. Unfortunately the radial velocity data have not been published yet. RS Orionis The O-C diagram of RS Ori is very interesting: possibly shows both a light-time effect and a phase jump. The photoelectric and the reliable earlier photographic observations have been converted into O-C residuals (see Table 82) using the ephemeris: C = 2442820.769 + 7.566841d*E (61) +-.009 +-.000014 Table 82. O-C residuals for RS Ori Norm.max. E O-C W Reference JD2400000+ 16533.795 -3474 0.232 1 Kukarkina (1955) 21936.353 -2760 0.145 1 Jordan (1929) 25326.322 -2312 0.089 1 Puchinskas (1962) 26393.242 -2171 0.085 1 Martynov (1951) 27233.264 -2060 0.187 1 Martynov (1951) 27581.209 -2014 0.058 1 Puchinskas (1962) 27982.333 -1961 0.139 1 Martynov (1951) 28565.060 -1884 0.219 1 Martynov (1951) 29064.379 -1818 0.127 1 Martynov (1951) 29306.578 -1786 0.187 1 Koshkina (1963) 29798.322 -1721 0.086 1 Martynov (1951) 30751.793 -1595 0.135 1 Martynov (1951) 31039.385 -1557 0.187 1 Kukarkina (1955) 33892.019 -1180 0.122 1 Koshkina (1963) 33960.037 -1171 0.039 1 Solov'yov (1956) 35178.286 -1010 0.026 2 Irwin (1961) 35208.500 -1006 -0.027 2 Walraven et al. (1958) 36192.193 -876 -0.023 3 Bahner et al. (1977) 36282.987 -864 -0.031 1 Puchinskas (1962) 36835.395 -791 -0.003 3 Weaver et al. (1960) 37047.280 -763 0.011 3 Mitchell et al. (1964) 38076.401 -627 0.041 1 Fridel' (1971) 40777.748 -270 0.026 3 Pel (1976) 42820.794 0 0.025 3 Szabados (1980) 44440.057* 214 -0.016 3 Moffett & Barnes (1984) 44644.241* 241 -0.137 1 present paper 44795.791* 261 0.076 1 Eggen (1985) 44969.752* 284 0.000 3 Moffett & Barnes (1984) Figure 53. Upper panel: O-C diagram of RS Ori Lower panel: gamma-velocities for the same Cepheid Table 83. gamma-velocities of RS Ori J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 23814 66 42 3 5 Joy (1937) 26029 400 45 2 8 Joy (1937) 46866 1 30 2 2 Samus (1990) The O-C graph in Figure 53 shows a phase jump between J.D. 2431000 and 2435000 (as already noted in Paper II). In addition to this phase shift (of -0.13 day) a long-term sinusoidal pattern of the O-C residuals is also suspected (see the free-hand dashed lines in Figure 53). If these waves are attributed to the orbital motion of the Cepheid component, the phasing of the gamma-velocity variations (see Table 83 and the lower panel of Figure 53) is in accord with the expected tendency. The wave and the rejump to the earlier pulsation period has to be confirmed. Before J.D. 2431000 the period was 7.566830 +- 0.000034 days. An attempt was made to approximate the whole interval of observations with a single line. In that case, however, the amplitude of the sinusoidal term would be too large, therefore incompatible with the light-time interpretation. The gamma-velocities determined for RS Ori are not accurate enough, because no well covered pulsational radial velocity curve has been obtained yet. Nevertheless, the variability of the gamma-velocity cannot be doubted. Duplicity of RS Ori has been confirmed by recent IUE observations (Evans et al., 1990a), the spectral type of the companion being B8V - B9V. RS Orionis is a very promising target for the observers. GQ Orionis Using the photoelectric observations published in the last decade, the pulsation period of GQ Ori can be refined. The O-C residuals listed in Table 84 (see also Figure 54) have been obtained with the help of the following ephemeris: C = 2442798.377 + 5.616283d*E (62) +-.011 +-.000058 The only available series of radial velocity observations (Barnes et al., 1988) is not enough for studying the duplicity of GQ Ori. Figure 54. O-C diagram of GQ Ori Table 84. O-C residuals for GQ Ori Norm.max. E O-C W Reference JD2400000+ 41066.557 -201 0.053 2 Pel (1976) 41368.070 -166 -0.004 3 Wachmann (1976) 42798.348 0 -0.029 3 Szabados (1980) 44392.411* 185 0.022 3 Moffett & Barnes (1984) 44599.171* 209 -0.009 2 Eggen (1985) 44986.928* 254 0.015 3 Moffett & Barnes (1984) The O-C residuals listed in Table 85 have been computed with the elements: C = 2443839.303 + 11.129319d*E (63) +-.008 +-.000019 The plot of the O-C residuals in Figure 55 shows that a phase shift might have occurred. This has been already pointed out in Paper III. The pulsation period was 11.129027 +- 0.000183 days before J.D. 2426000, and a Figure 55. Upper panel: O-C diagram of SV Per Lower panel: gamma-velocities for the same Cepheid Table 85. O-C residuals for SV Per Norm.max. E O-C W Reference JD2400000+ 17930.893 -2328 0.645 1 Enebo (1908) 18298.108 -2295 0.592 1 Enebo (1909) 18977.140 -2234 0.736 1 Enebo (1914) 20112.339 -2132 0.744 1 Enebo (1914) 26288.996 -1577 0.629 1 Kukarkin (1940) 26377.692 -1569 0.291 1 Rugemer (1932) 30027.889 -1241 0.071 Mergentaler (1948) 33232.980 -953 -0.082 2 Eggen et al. (1957) 35848.470 -718 0.018 3 Bahner & Mavridis (1977) 36260.245 -681 0.008 3 Bahner & Mavridis (1977) 37595.773 -561 0.018 2 Mitchell et al. (1964) 39075.985 -428 0.030 3 Takase (1969) 39209.541 -416 0.035 3 Wamsteker (1972) 41913.906 -173 -0.025 3 Vasil'yanovskaya (1977) 43839.246 0 -0.057 3 Szabados (1981) 44473.663* 57 -0.011 3 Moffett & Barnes (1984) 44529.365* 62 0.044 2 Eggen (1983b) 44651.810* 73 0.067 2 present paper 44963.375* 101 0.011 3 Moffett & Barnes (1984) 45686.725* 166 -0.045 3 Berdnikov (1986) phase shift of about -0.3 day might occur between J.D. 2426000 and 2430000. Since most of the early O-C residuals are based on visual observations, the phase jump cannot be determined as clearly as in the cases when more accurate observations are also available. Table 86. gamma-velocities of SV Per J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 23635 264 -9.4 3.2 3 Joy (1937) 24822 18 -8.6 4.5 2 Joy (1937) 25413 157 -10.3 2.6 4 Joy (1937) 26572 30 -6.3 3.2 3 Joy (1937) 41252 1 1.8 0.1 3 Lloyd Evans (1984) 44586 290 -1.7 0.9 20 Barnes et al. (1988) The gamma-velocities listed in Table 86 confirm the previous conclusion drawn by Lloyd Evans (1984) about the spectroscopic binary nature of SV Per. The blue companion suspected from photometry (Madore, 1977) has been discovered in the ultraviolet spectrum of this Cepheid (Bohm-Vitense and Proffitt, 1985). The orbital period might not be very short, because Gieren and Brieva (1990) found no evidence for variable gamma-velocity between 1971 and 1987. VX Persei The old O-C residuals corrected according to the new normal curve, supplemented with the more recent residuals (see Table 87) indicate a changing period (see Figure 56). The new ephemeris used in the present study is as follows: C = 2443759.184 + 10.886972d*E (64) +-.017 +-.000142 Figure 56. Upper panel: O-C diagram of VX Per Lower panel: gamma-velocities for the same Cepheid Table 87. O-C residuals for VX Per Norm.max. E O-C W Reference JD2400000+ 36811.935 -638 -1.361 3 Oosterhoff (1960) 36822.785 -637 -1.398 3 Weaver et al. (1960) 43759.125 0 -0.059 3 Szabados (1981) 44358.001* 55 0.034 3 Moffett & Barnes (1984) 44532.214* 71 0.055 3 Eggen (1983b) 44651.885* 82 -0.031 2 present paper 44956.767* 110 0.016 3 Moffett & Barnes (1984) 46284.936* 232 -0.026 3 Berdnikov (1987) Table 88. gamma-velocities of VX Per J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25113 193 -33.3 3.2 3 Joy (1937) 26626 477 -32.4 2.6 4 Joy (1937) 44386 178 -37.0 1.0 18 Barnes et al. (1988) 44948 57 -38.4 2.0 5 Barnes et al. (1988) 46767 64 -35.3 0.1 56 Coker et al. (1989) 47004 15 -34.8 0.3 7 Metzger et al. (1990) 47124 63 -35.4 0.2 6 Coker et al. (1989) Before J.D. 2443000 the pulsation period was longer, but an accurate value of the period for the previous epochs cannot be determined due to the paucity of data. A parabolic O-C graph, i.e. a continuously decreasing period is also possible, and photometric observations to be obtained in the near future will give a definitive answer for this problem. The gamma-velocity of VX Per can be considered as stable (see the values listed in Table 88). No other evidence exists for the duplicity of this Cepheid. AS Persei The recent high quality photoelectric observations (especially those obtained by Moffett and Barnes, 1984) do not support the hypothesis raised in Paper I concerning the changing pulsational amplitude of AS Per. The O-C residuals listed in Table 89 (see also Figure 57) have been obtained using the elements: C = 2441723.941 + 4.972540d*E (65) +-.004 +-.000007 The pulsation period of AS Per has remained constant since the beginning of the photoelectric observations. The available radial velocity observations (Joy, 1937) are not suffi- cient for the reliable determination of the gamma-velocity, therefore new radial velocity observations would be valuable. Figure 57. O-C diagram of AS Per Table 89. O-C residuals for AS Per Norm.max. E O-C W Reference J.D. 2400000+. 36816.019 -987 -0.025 2 Weaver et al. (1960) 36821.041 -986 0.024 2 Oosterhoff (1960) 39252.614 -497 0.025 2 Takase (1969) 41723.922 0 -0.019 3 Szabados (1977) 43484.207* 354 -0.013 3 present paper 43688.080* 395 -0.014 2 Henden (1979) 44473.768* 553 0.012 3 Moffett & Barnes (1984) 44971.024* 653 0.014 3 Moffett & Barnes (1984) AW Persei This Cepheid, belonging to a wide binary system, has become a popular object among the observers. The behaviour of AW Per is best described by Table 90. O-C residuals for AW Per Norm.max. E O-C W Reference JD2400000+ 29070.812 -2110 -0.021 1 Opolski (1948) 32865.073 -1523 0.094 1 Erleksova (1961) 35463.438 -1121 0.083 1 Erleksova (1961) 36109.735 -1021 0.018 3 Bahner & Mavridis (1977) 36426.431 -972 -0.003 1 Erleksova (1961) 36820.711 -911 -0.004 3 Oosterhoff (1960) 36827.181 -910 0.002 3 Weaver et al. (1960) 39503.063 -496 -0.055 3 Wamsteker (1972) 40155.863* -395 -0.081 1 Feltz & McNamara (1980) 40996.175* -265 -0.040 2 Feltz & McNamara (1980) 40996.204 -265 -0.011 2 Evans (1976) 42709.062 0 -0.013 3 Szabados (1980) 43704.490* 154 0.017 3 Moffett & Barnes (1984) 44079.371* 212 0.008 3 Moffett & Barnes (1984) 44641.665* 299 -0.033 1 present paper 46477.402* 583 0.035 2 present paper 47311.211* 712 0.037 2 present paper Figure 58. Upper panel: O-C diagram of AW Per Lower panel: gamma-velocities for the same Cepheid Table 91. gamma-velocities of AW Per J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 26981 188 14.8 1.6 9 Joy (1937) 37247 33 -2.1 0.9 8 Miller & Preston (1964b) 37798 223 -4.5 0.9 9 Miller & Preston (1964b) 40214 43 19.7 1.2 2 Welch & Evans (1989) 40971 4 12.9 0.4 3 Evans (1976) 42989 188 15.3 0.2 7 Griffin (1982) * 43155 2 18.5 1.0 2 McNamara & Chapman (1977) 43408 49 12.3 2.0 5 Wilson et al. (1989) 43559 15 6.0 1.0 2 Evans (1983) 43639 179 12.9 0.2 11 Griffin (1982) * 43821 1 -0.3 0.8 1 Beavers & Eitter (1986) 43962 220 12.3 0.8 25 Barnes et al. (1987) 44549 169 11.5 0.2 14 Griffin (1982) * 44836 159 13.4 0.4 5 Evans (1983) 45036 373 8.0 1.6 6 Barnes et al. (1987) 45610 1 13.9 1.2 1 Welch & Evans (1989) 46639 118 10.5 0.4 12 Welch & Evans (1989) 46727 1 7.5 3.4 2 present paper 47091 4 8.8 1.2 2 Welch & Evans (1989) * Observer: T. Lloyd Evans Welch and Evans (1989), and the additional photometric and spectroscopic data analysed here confirm their results. The O-C residuals listed in Table 90 have been calculated with the formula: C = 2442709.075 + 6.463622d*E (66) +-.007 +-.000009 The plot of the O-C residuals in Figure 58 clearly shows the wave-like pattern due to the orbital motion. The light-time effect keeps on being consistent with the phase of the gamma-velocity variation. In addition to the radial velocity data analysed by Welch and Evans (1989), Table 91 also lists some other gamma-velocity values that have not been used for deriving the orbit of AW Persei, including that obtained from Lloyd Evans' radial velocity measurements (Griffin, 1982), and from the two spectra taken by the author at Rozhen Observatory (see Table 109). The gamma-velocity values plotted in the lower panel of Figure 58 well demonstrate the long orbital period: 13100 +- 100 days, as determined by Welch and Evans (1989). In view of its importance, regular photometric observations of AW Per will continue at Konkoly. V440 Persei Due to the short time-base of the available photometric observations, the shape of the O-C graph cannot be determined reliably. The O-C residuals listed in Table 92 have been obtained using the linear ephemeris: C = 2444551.137+ 7.572498d*E (67) +-.014 +-.000074 Further observations are necessary to decide whether an additional parabolic term would give a better fit when predicting the forthcoming light maxima (cf. Figure 59). Figure 59. Upper panel: O-C diagram of V440 Per Lower panel: gamma-velocities for the same Cepheid Table 92. O-C residuals for V440 Per Norm.max. E O-C W Reference JD2400000+ 41219.322* -440 0.084 2 Burki & Benz (1982) 43483.394 -141 -0.021 3 Szabados (1980) 44240.668* -41 0.003 3 Burki & Benz (1982) 44528.363* -3 -0.057 3 Eggen (1985) 44551.047* 0 -0.090 3 Burki & Benz (1982) 44869.217* 42 0.035 3 Arellano Ferro (1984) 46724.514* 287 0.070 3 Arellano Ferro et al. (1987) Table 93. gamma-velocities of V440 Per J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 22863 107 -27.4 0.9 6 Abt (1970) 23699 28 -23.7 2.5 2 Plaskett (1934) 43796 126 -25.6 0.1 18 Burki & Benz (1982) 44202 24 -25.6 0.1 23 Burki & Benz (1982) 44525 1 -22.4 0.8 2 Beavers & Eitter (1986) 44559 67 -25.6 0.1 19 Burki & Benz (1982) 44712 136 -25.8 0.4 15 Arellano Ferro (1984) The gamma-velocities of V440 Per are listed in Table 93. Although the gamma-velocity seems to be constant, the variable star may have a companion, but the inclination of the orbit is unfavourable for detecting the orbital motion. In addition to the fact that Usenko (1990b) assumes a B8 type photometric companion, the suspicion supporting the existence of the companion is based on the relatively large radial velocity amplitude, as compared with the amplitude of the light variation. It is obvious that a companion star tends to diminish the photometric amplitude of the variable, and does not influence the pulsational radial velocity amplitude (i.e. having removed the orbital effect). The binary Cepheids usually have a larger ratio of radial velocity amplitude per light variation amplitude (Szabados, unpublished). A detailed analysis of this effect is in progress. S Sagittae The reliable earlier O-C residuals taken from Paper II have been supplemented with the O-C values derived from the more recently published photoelectric observations (see Table 94). The O-C residuals (partly plotted in Figure 60) have been approximated with a parabola instead of two interesting straight lines, as suggested in Paper II. The O-C residuals in Table 94 have been computed with the formula: C = 2442678.821+ 5.382146d*E (68) +-.021 +-.000029 Table 94. O-C residuals for S Sge Norm.max. E O-C W Reference JD2400000+ 09829.793 -3919 0.602 1 Gore (1886) 09888.258 -3912 0.392 1 Gore (1886) 10164.999 -3879 0.522 1 Gore (1887) 10567.248 -3831 0.428 1 Gore (1888) 11263.052 -3748 0.514 1 Gore (1890) 11615.078 -3706 0.490 1 Gore (1891) 11975.160 -3663 0.140 1 Markwick (1892) 14196.461 -3398 0.172 1 Pickering (1904) 14934.134 -3310 0.216 1 Prittwitz (1901) 16082.542 -3173 0.270 1 Tass (1925) 16761.439 -3092 0.213 1 Lau (1907) 16803.375 -3087 0.239 1 Tass (1925) 17062.887 -3056 -0.096 1 Wilkens (1906) 17632.674 -2988 -0.295 1 Jordan (1919) 17850.862 -2962 -0.043 1 Hertzsprung (1909) 17851.029 -2962 0.124 1 Nijland (1923) 17851.054 -2962 0.149 1 Zeipel (1908) 18035.437 -2940 0.125 1 Tass (1925) 18152.813 -2926 0.151 1 Nijland (1923) 18521.561 -2882 0.085 1 Nijland (1923) 18806.555 -2848 0.086 1 Nijland (1923) 18957.443 -2830 0.095 2 Hertzsprung (1917) 19317.859 -2787 0.079 2 Hertzsprung (1917) 21061.385 -2579 0.119 1 Luyten (1922) 21488.867 -2528 0.111 1 Lacchini (1921) 21748.891 -2497 0.289 1 Luyten (1922) 22192.987 -2444 0.131 1 Leiner (1926) 22897.156 -2360 0.200 1 Eaton (1920,1921,1922) Walker (1921,1922) 23316.013 -2310 -0.051 1 Nielsen (1927c) 23333.038 -2308 0.210 1 AFOEV (1922,1923) 24213.006 -2203 0.053 1 Parenago (1938) 24690.941 -2146 0.205 1 Leiner (1938) 24774.587 -2136 0.030 1 Kukarkin (1940) 25134.962 -2093 -0.027 1 Hellerich (1935) 25386.645 -2063 0.191 1 Leiner (1938) 25445.085 -2056 -0.044 1 Zverev (1936) 25453.702 -2055 0.191 1 Kukarkin (1940) 25797.187 -2014 0.008 1 Zverev (1936) 25847.689 -2008 0.217 1 Leiner (1938) 26182.943 -1968 0.185 1 Leiner (1938) 26384.157 -1944 0.228 1 Parenago (1938) 26451.214 -1936 0.228 1 Kukarkin (1940) 26476.126 -1933 -0.007 1 Zverev (1936) 26794.741 -1895 0.087 1 Kox (1935) 26920.633 -1880 0.246 1 Florya & Kukarkina (1953) 27591.056 -1800 0.010 1 Krebs (1935) 28001.975 -1751 0.292 1 Krebs (1936) 29091.599 -1621 0.237 1 Leiner (1938) 29141.833 -1615 0.178 3 Bennett (1939) 33131.605 -1139 0.048 3 Eggen (1951) 33198.520 -1131 -0.094 1 Solov'yov (1959) 34036.880 -1031 0.052 1 Solov'yov (1959) 34598.351 -964 -0.081 1 Solov'yov (1959) Table 94. (cont.) Norm.max. E O-C W Reference JD2400000+ 34615.241 -962 0.044 3 Szabados (1980) 35285.815 -882 0.047 2 Irwin (1961) 35403.191 -868 0.073 1 Solov'yov (1959) 35730.121 -829 0.099 3 Prokof'yeva (1961) 35730.154 -829 0.132 1 Solov'yov (1959) 36048.551 -791 0.007 1 Solov'yov (1959) 36190.964 -774 -0.076 1 Latyshev (1969) 36207.768 -772 -0.036 1 Svolopoulos (1960) 36450.962 -743 0.075 1 Solov'yov (1959) 36459.445 -742 0.176 1 Solov'yov (1959) 36509.621 -736 0.059 3 Walraven et al. (1958) 36718.940 -711 -0.175 1 Azarnova (1960b) 37213.656 -652 -0.006 3 Mitchell et al. (1964) 37917.700 -568 -0.062 3 Walraven et al. (1964) 39040.928 -434 -0.042 3 Wisniewski & Johnson (1968) 40440.802* -267 0.014 3 Feltz & McNamara (1980) 40834.696 -220 -0.053 2 Evans (1976) 41220.269* -174 -0.059 2 Feltz & McNamara (1980) 42595.144 -10 0.144 1 Berdnikov (1977) 42678.783 0 -0.038 3 Szabados (1980) 43340.987* 79 -0.024 3 Moffett & Barnes (1984) 43835.505* 138 -0.052 3 Moffett & Barnes (1984) 44808.023* 254 0.137 1 present paper 46643.574* 473 -0.002 2 present paper Figure 60. O-C diagram of S Sge The continuous period increase is given by the equation: P = 5.382146d + 4.4d*10^-8*E (69) +-.000029 +-1.6 Future photoelectric observations are needed to decide whether the deviations from the parabola are systematic or random. It is clear, however, that the light-time effect expected from the orbital motion is much less than the detection limit. S Sge is one of the best studied binary Cepheids, therefore no attempt was made here for determining the orbital elements. A single new radial velocity value obtained with the 2m telescope in Rozhen is, however, listed in Table 109. The most recent results concerning the duplicity of S Sge are published by Slovak and Barnes (1987) (Porb = 676.2 days), Evans et al. (1989), and Slovak et al. (1989). The two latter papers put forward evidence for a third component in the system, what makes S Sge an even more interesting object for future observations. SW Tauri The previously determined O-C residuals (see Paper I) have been corrected according to the new normal light curve based on the superior observations made by Moffett and Barnes (1984). The corrected and the newly determined O-C residuals are all listed in Table 95. The plot of these residuals in Figure 61 shows a parabolic shape instead of the occurrence of a single period change suggested in Paper I. The new ephemeris used when determining the O-C values is as follows: C = 2441687.762 + 1.583577d*E (70) +-.003 +-.000001 The continuous period decrease can be computed from the formula: P = 1.583577d +- 7.13d*10^-9*E (71) +-.000001 +-.54 This slight and smooth period change is atypical of the short period Population II Cepheids. Figure 61. Upper panel: O-C diagram of SW Tau Lower panel: gamma-velocities for the same Cepheid Table 95. O-C residuals for SW Tau Norm.max. E O-C W Reference JD2400000+ 30434.681 -7106 -0.183 1 Solov'yov (1957) 32354.052 -5894 -0.107 1 Borzdyko (1965) 33638.337 -5083 -0.103 2 Eggen et al. (1957) 34377.911 -4616 -0.059 1 Borzdyko (1965) 35879.185 -3668 -0.017 1 Borzdyko (1965) 37465.873 -2666 -0.073 1 Mandel' (1970) 39059.018 -1660 -0.006 3 Milone (1970) 39078.014 -1648 -0.013 3 Wamsteker (1972) 41687.762 0 0.000 3 Szabados (1977) 42455.800* 485 0.003 3 Dean et al. (1977) 43166.821* 934 -0.002 3 Stobie & Balona (1979) 43176.324 940 0.000 2 Szabados (1977) 43630.795* 1227 -0.016 2 Henden (1979) 43675.141* 1255 -0.010 2 Diethelm & Tammann (1982) 44519.188* 1788 -0.010 3 Moffett & Barnes (1984) 44994.253* 2088 -0.018 3 Moffett & Barnes (1984) 45966.558* 2702 -0.029 2 Diethelm (1986) 46337.110* 2936 -0.034 2 Wallerstein (1987) Table 96. gamma-velocities of SW Tau J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 24993 176 15.3 2.6 4 Joy (1937) 27343 174 14.6 2.0 6 Joy (1937) 28175 59 16.8 2.3 5 Joy (1937) 43139 26 11.5 0.6 17 Stobie & Balona (1979) 43526 1 8.0 2.5 1 Stobie & Balona (1979) 44187 29 8.6 1.6 7 Barnes et al. (1988) 44555 49 12.2 1.3 10 Barnes et al. (1988) 44948 51 8.0 1.8 6 Barnes et al. (1988) 47793 1 10.4 1.2 1 Samus (1990) The gamma-velocity of SW Tau seems to be variable (see Table 96 and the lower panel of Figure 61). Nevertheless, further radial velocity observations are necessary to confirm the duplicity of this variable star. SZ Tauri A rejump to an earlier pulsation period was reported in Paper I. Since then a new period change has occurred (Trammell, 1987), and interestingly enough, the new period is almost identical with that valid during the interval 1920 - 1960. The photoelectric O-C residuals listed in Table 97 confirm the new phase jump, though this event took place so slowly that the intermediate period can well be determined (see Figure 62). The current ephemeris used for calculating the O-C residuals is as follows: C = 2441659.262 + 3.149138d*E (72) +-.032 +-.000043 Figure 62. Upper panel: O-C diagram of SZ Tau Lower panel: gamma-velocities for the same Cepheid Table 97. O-C residuals for SZ Tau Norm.max. E O-C W Reference JD2400000+ 32852.379 -2797 1.256 2 Eggen (1951) 35541.659 -1943 1.172 1 Walraven et al. (1958) 37619.809 -1283 0.891 3 Mitchell et al. (1964) 37962.988 -1174 0.814 1 Williams (1966) 38529.659 -994 0.640 3 Wisniewski & Johnson (1968) 39055.461 -827 0.536 3 Milone (1970) 39077.600 -820 0.631 3 Wamsteker (1972) 39807.965 -588 0.396 3 Szabados (1977) 40147.996* -480 0.032 1 Feltz & McNamara (1980) 40991.677* -212 0.032 1 Feltz & McNamara (1980) 41659.204 0 -0.058 3 Szabados (1977) 43520.462* 591 0.059 3 Moffett & Barnes (1984) 43926.666* 720 0.025 3 Moffett & Barnes (1984) 44647.837* 949 0.043 1 present paper 46845.772* 1647 -0.120 1 Trammell (1987) The earlier values of the pulsation period have not been determined again, these values are taken from Paper I: before J.D. 2418500 P = 3.14839 days, between J.D. 2425500 and 2436300 P = 3.149057 days, between J.D. 2436300 and 2441300 P = 3.148380 days, after J.D. 2441300 P = 3.149138 days. Because of the stepwise O-C graph, i.e. the phenomenon of the phase jump, SZ Tau probably belongs to a binary system, as well. The existence of a bright blue companion is, however, doubtful (see Leonard and Turner, 1986, and the references therein). As can be seen in the lower panel of Table 98. gamma-velocities of SZ Tau J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 20095 49 -3.7 0.6 30 Haynes (1914) 21784 236 -2.5 1.2 4 Abt (1970) 40972 10 1.4 0.8 5 Schmidt (1974) 43449 59 -0.3 1.4 9 Wilson et al. (1989) 44011 220 -5.5 0.8 25 Barnes et al. (1987) 45311 147 -1.3 1.5 8 Barnes et al. (1987) 45721 8 -0.1 0.1 36 Gieren (1985) 46866 1 0.9 0.5 2 Samus (1990) Figure 62, and in the data listed in Table 98, the gamma-velocity of SZ Tau might be variable. The spectroscopic binary nature of SZ Tau, however, needs confirmation. SZ Tau is an important object because of one more reason, viz. its possible membership in the open cluster NGC 1647 (Walker, 1987; Gieren, 1988). S Vulpeculae S Vul is one of the longest period Cepheids in our Galaxy. Its pulsation period has been strongly varying (Mahmoud and Szabados, 1980), and the latest period change just noticeable in 1980 now can be traced. The O-C residuals listed in Table 99, and shown plotted in Figure 63 have been obtained with the elements: C = 2444147.692+ 68.500d*E (73) +-.518 +-.027 Figure 63. O-C diagram of S Vul Table 99. O-C residuals for S Vul Norm.max. E O-C W Reference JD2400000+ 40017.349 -61 48.157 3 Fernie (1970) 41975.061* -32 19.369 1 Schmidt (1976) 42585.601* -23 13.409 1 Dawson (1979) 43738.161 -6 1.469 1 Turner (1980) 44147.253 0 -0.439 3 Mahmoud & Szabados (1980) 44422.573 4 0.881 2 Mahmoud & Szabados (1980) 44832.141* 10 -0.551 1 present paper 45584.257* 21 -1.935 3 Berdnikov & Ivanov (1986) 45861.682* 25 1.490 1 Berdnikov & Ivanov (1986) 46272.203* 31 1.011 3 Berdnikov & Ivanov (1986) The available radial velocity data do not permit even the reliable determination of the gamma-velocity, but as a matter of fact, no evidence can be found in the literature regarding the duplicity of S Vul. T Vulpeculae A new normal light curve has been determined on the basis of the photoelectric observations made by Moffett and Barnes (1984). The new O-C residuals (listed in Table 100) have been determined with the help of this normal curve, and the O-C residuals published earlier in Paper I have been corrected accordingly. The best linear fit to the data points after J.D. 2434500 is as follows: C = 2441705.127 + 4.435453d*E (74) +-.007 +-.000009 Figure 64. Upper panel: O-C diagram of T Vul Lower panel: gamma-velocities for the same Cepheid Table 100. O-C residuals for T Vul Norm.max. E O-C W Reference JD2400000+ 20001.972 -4893 -0.483 Hertzsprung (1919) 20747.181 -4725 -0.431 Hertzsprung (1919) 24375.483 -3907 -0.329 Huffer (1928a) 24721.463 -3829 -0.314 Huffer (1928a) 25116.367 -3740 -0.166 Hellerich (1935) 26859.398 -3347 -0.268 Kox (1935) 32967.242 -1970 -0.043 Eggen (1951) 34595.189 -1603 0.093 1 Szabados (1977) 35362.401 -1430 -0.028 1 Walraven et al. (1958) 35757.181 -1341 -0.004 3 Prokof'yeva (1961) 36098.640 -1264 -0.074 2 Svolopoulos (1960) 37212.028 -1013 0.015 2 Mitchell et al. (1964) 37562.390 -934 -0.024 3 Johansen (1971) 37939.534 -849 0.107 1 Williams (1966) 38649.098 -689 -0.002 3 Johansen (1971) 38733.365 -670 -0.008 2 Wisniewski & Johnson (1968) 40254.769* -327 0.035 2 Feltz & McNamara (1980) 40853.449* -192 -0.071 1 Evans (1976) 41128.537* -130 0.019 2 Feltz & McNamara (1980) 41705.136 0 0.009 3 Szabados (1977) 43359.552* 373 0.001 3 Moffett & Barnes (1984) 43874.070* 489 0.006 3 Moffett & Barnes (1984) 44472.831* 624 -0.019 3 Berdnikov & Bogdanov (1987) Table 101. gamma-velocities of T Vul J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 16683 2 9.0 3.0 2 Frost (1904) 16699 19 -1.5 1.5 5 Albrecht (1907) 16765 11 -3.8 3.0 2 Albrecht (1907) 17093 18 1.0 1.5 5 Albrecht (1907) 17151 19 -1.4 0.6 24 Albrecht (1907) 17214 3 -3.5 3.0 2 Albrecht (1907) 17438 7 -0.8 0.8 14 Albrecht (1907) 21796 2 -7.5 2.0 2 Abt (1973) 22166 68 -2.5 2.0 2 Abt (1973) 23622 11 0.8 3.0 2 Harper (1934) 26179 10 0.9 0.9 12 Lust-Kulka (1954) 26220 14 1.8 0.8 16 Lust-Kulka (1954) 27995 1 5.8 3.0 1 Young (1939) 28450 1 -6.4 1.4 3 Abt (1973) 28760 32 -4.5 2.1 3 Young (1939) 40825 24 -3.2 0.3 4 Evans (1976) 41336 11 -2.8 0.4 3 Evans (1976) 43293 1 2.8 4.0 2 Wilson et al. (1989) 43327 33 -2.4 0.3 3 Evans & Lyons (1986) 43377 46 -3.2 0.4 5 Beavers & Eitter (1986) 43382 4 -3.2 0.9 21 Wilson et al. (1989) 43500 5 -0.7 2.0 5 Wilson et al. (1989) 43735 69 1.1 1.2 11 Barnes et al. (1987) 44134 81 -2.2 1.1 12 Barnes et al. (1987) 44136 36 -2.8 0.2 4 Evans & Lyons (1986) 44811 1 -3.2 0.4 1 Evans & Lyons (1986) 45539 24 -1.6 0.2 4 Evans & Lyons (1986) Before J.D. 2434500 the pulsation period was somewhat longer (see Figure 64): P = 4.435589 days, as determined in Paper I. Moreover, it cannot be excluded that the period of pulsation before J.D. 2419000 practically coincided with the recent value, i.e. a phase slide occurred between J.D. 2419000 and 2434500. This phenomenon cannot be followed in Figure 64, because the early visual observations have not been analysed here. Another kind of phase jump can, however, be suspected in the recent part of the O-C graph. In Figure 64 dashed lines indicate these assumed phase jumps. The quasi-period in the recurrence of these jumps is the same as the value of the orbital period of T Vul discussed below. This phenomenon is similar to that observed in Y Oph (see Paper IV, p. 39). The variation in the gamma-velocity exceeds the limit that could be attributed to the observational uncertainty. The considerably large number of the gamma-velocities listed in Table 101 (and partly plotted in the lower panel of Figure 64) has been analysed for the possible periodicity. A reasonably good "orbital" radial velocity curve could be obtained at a period of 1745 days. This phase diagram is plotted in Figure 65. The only strongly deviating gamma-velocity is the point determined from Frost's (1904) two radial velocity measurements, being the earliest velocity data for T Vul. Interestingly enough, Kovacs et al. (1990) also found a long periodicity in the radial velocity data (their sample was a subset of the data studied here), but the period of 738 days discovered by them cannot be revealed from the present sample. The strong coincidence of the 1745 day period with the cycle length of the subtle phase jumps in the O-C diagram Figure 65. gamma-velocity values of T Vul folded with the 1745 day period confirm the author's belief that this longer period represents the orbital period of the binary system containing T Vul. The presence of a companion star has already been suspected by Kurochkin (1966). Extensive photometric and spectroscopic observations would be necessary to obtain a more reliable picture on the duplicity of T Vul. U Vulpeculae The O-C diagram covering almost a century was approximated with a single straight line in Paper II. The photoelectric O-C residuals supplemented with some more data published in the eighties (see Table 102 and Figure 66) are better represented by two linear sections. It has to be noted that the extrapolation of this alternating period change towards the earlier visual observations also gives a reasonably good fit to the O-C residuals. The O-C residuals listed in Table 102 have been obtained with the elements: C = 2442526.312+ 7.990821d*E (75) +-.006 +-.000026 The gamma-velocities of U Vul show an intrinsic variation (see Table 103 and the lower panel of Figure 66) which can be explained with the duplicity of U Vul. The existence of a companion was already suspected by Kurochkin (1966). The gamma-velocities derived from Sanford's (1928) radial velocity data indicate that the orbital period might not be too long (i.e. much longer than 1000 days). The formal period search routine applied to Figure 66. Upper panel: O-C diagram of U Vul Lower panel: gamma-velocities for the same Cepheid Table 102. O-C residuals for U Vul Norm.max. E O-C W Reference JD2400000+ 33704.781 -1104 0.335 1 Chuprina (1952) 34591.618 -993 0.191 3 Szabados (1980) 35638.397 -862 0.173 3 Walraven et al. (1958) 36125.772 -801 0.108 3 Bahner & Mavridis (1971) 36781.008 -719 0.096 3 Weaver'et al. (1960) 37244.483 -661 0.104 3 Mitchell et al. (1964) 40121.046 -301 -0.029 3 Asteriadis et al. (1977) 40208.971* -290 -0.003 1 Feltz & McNamara (1980) 40840.247 -211 -0.002 3 Evans (1976) 41199.889* -166 0.053 2 Feltz & McNamara (1980) 42526.328 0 0.016 3 Szabados (1980) 44292.262* 221 -0.021 3 Moffett & Barnes (1984) 44923.562* 300 0.004 3 Moffett & Barnes (1984) Table 103. gamma-velocities of U Vul J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 21759 28 -13.5 1.7 4 Sanford (1928) 24745 39 -7.9 0.9 13 Sanford (1928) 25111 48 -12.5 0.9 13 Sanford (1928) 31776 185 -10.5 1.2 7 Sanford (1951) 32494 128 -12.1 1.2 7 Sanford (1951) 41067 116 -14.7 0.3 6 Evans (1976) 44369 219 -13.0 1.1 15 Barnes et al. (1988) 44877 76 -10.5 1.6 7 Barnes et al. (1988) 47365 1 -6.1 0.4 1 Samus (1990) the gamma-velocity values of U Vul resulted in the orbital period of 868 days. This value, however, cannot be accepted without reservation. The determination of the true value of the orbital period is not possible without performing new radial velocity measurements. Photometric observations would be necessary, as well, in order to study whether the alternating period changes represent a kind of phase jump (or phase slide) characteristic of binary Cepheids. It is clear, however, that the alternating period changes cannot be replaced with a sinusoidal wave caused by the light-time effect, because the gamma-velocity data indicate an oscillation in the O-C diagram that would hardly be detected. X Vulpeculae The O-C residuals of X Vul listed in Table 104 have been obtained using the ephemeris: C = 2442665.932 + 6.319490d*E (76) +-.012 +-.000022 Instead of a single straight line, the O-C plot has been approximated with two sections and a phase jump in between (see Figure 67). This phase jump, however, has to be confirmed. The gamma-velocity of X Vul varies, and this Cepheid probably belongs to a long period spectroscopic binary (see Table 105 and the lower panel of Figure 67). Moffett and Barnes (1987) also noted the discrepancy between the gamma-velocity values determined at two epochs, while Janot-Pacheco (1976) suspected a photometric companion. Figure 67. Upper panel: O-C diagram of X Vul Lower panel: gamma-velocities for the same Cepheid Table 104. O-C residuals for X Vul Norm.max. E O-C W Reference JD2400000+ 17470.166 -3987 0.041 1 Luizet (1907) 26481.643 -2561 -0.075 1 Kukarkin (1940) 28314.212 -2271 -0.158 1 Azhusenis (1956) 28984.128 -2165 -0.108 1 Azhusenis (1956) 33117.093 -1511 -0.090 3 Eggen (1951) 33692.141 -1420 -0.115 1 Chuprina (1954b) 33868.946 -1392 -0.256 1 Chuprina (1954b) 35354.434 -1157 0.152 1 Walraven et al. (1958) 37205..847 -864 -0.046 3 Mitchell et al. (1964) 37357.564 -840 0.004 1 Boyko (1970) 39291.295 -534 -0.029 1 Boyko (1970) 41294.584* -217 -0.019 3 Feltz & McNamara (1980) 42665.923 0 -0.009 3 Szabados (1980) 44296.367* 258 0.007 3 Moffett & Barnes (1984) 44909.375* 355 0.024 3 Moffett & Barnes (1984) Table 105. gamma-velocities of X Vul J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 24748 2 -10.1 2.6 4 Joy (1937) 25490 233 -11.2 2.6 4 Joy (1937) 44504 273 -16.2 1.4 9 Barnes et al. (1988) 47361 1 -16.2 0.8 1 Samus (1990) SV Vulpeculae SV Vul is a very important Cepheid because of several reasons. Not only is SV Vul one of the longest period Cepheids in our Galaxy, but it is a possible member of the Vul OB1 association (see Walker, 1987 and the references therein). Table 106. O-C residuals for SV Vul Norm.max. E O-C W Reference JD2400000+ 23244.975 -454 -38.321 1 Kristensen (1923,1924) 23470.600 -449 -37.730 1 Leiner (1924) 23651.731 -445 -36.626 1 Zakharov (1924a,1924b) 23877.517 -440 -35.874 1 Kristensen (1926) 23923.562 -439 -34.836 1 Beyer (1930) 23967.607 -438 -35.798 1 Ahnert (1931) 24013.407 -437 -35.004 1 Zakharov (1928) 24148.197 -434 -35.235 1 Leiner (1929) 24329.463 -430 -33.996 1 Zakharov (1928) 24374.633 -429 -33.833 1 Beyer (1930) 24419.533 -428 -33.940 1 Nielsen (1927b) 24511.449 -426 -32.037 1 Ahnert (1931) 24736.039 -421 -32.481 1 Leiner (1929) 24736.624 -421 -31.896 1 Beyer (1930) 24781.299 -420 -31.228 1 Zakharov (1928) 25142.885 -412 -30.696 1 Beyer (1930) 25143.425 -412 -30.156 1 Ahnert (1931) 25369.366 -407 -29.249 1 Leiner (1929) 25550.091 -403 -28.552 1 Beyer (1930) 25865.787 -396 -27.903 1 Ahnert (1931) 25866.102 -396 -27.588 1 Kukarkin (1940) 25955.541 -394 -28.163 1 Zakharov (1954) 26180.987 -389 -27.751 1 Ahnert (1931) 26406.522 -384 -27.250 1 Zverev (1936) 26453.088 -383 -25.691 1 Terkan (1935) 26677.903 -378 -25.910 1 Kukarkin (1940) 26904.924 -373 -23.923 1 Florya & Kukarkina (1953) 27265.565 -365 -23.336 1 Florya & Kukarkina (1953) 28077.230 -347 -21.793 1 Nassau & Ashbrook (1942) 28754.097 -332 -20.028 1 Dziewulski & Iwanowska (1946) 32948.126 -239 -11.632 3 Eggen (1951) 33535.518 -226 -9.328 1 Chuprina (1953) 35340.791 -186 -4.327 1 Walraven et al. (1958) 37232.711 -144 -2.693 3 Mitchell et al. (1964) 37952.866 -128 -2.647 1 Williams (1966) 38268.336 -121 -2.224 3 Fernie et al. (1965) 40654.719* -68 -1.202 2 Feltz & McNamara (1980) 41329.910* -53 -1.113 2 Feltz & McNamara (1980) 43085.550 -14 -0.738 3 Fernie (1979a) 43625.769* -2 -0.600 3 Moffett & Barnes (1984) 43715.229 0 -1.154 3 Szabados (1981) 44075.401* 8 -1.036 3 Moffett & Barnes (1984) 44480.354* 17 -1.145 1 present paper 44525.619* 18 -0.886 2 Eggen (1983b) 45652.558* 43 0.883 3 Berdnikov (1986) 46283.295* 57 1.524 3 Berdnikov (1987) The behaviour of the period changes of this classical Cepheid continues to be very interesting. As it was pointed out in Paper III, erratic changes appear superimposed on the general parabolic trend of the O-C diagram. The new O-C residuals, as well as the earlier ones (corrected according to the new normal light curve) are listed in Table 106. The plot of these residuals shown in Figure 68 has been obtained with the elements: C = 2443716.383 + 45.0068d*E (77) +-.181 +-.0026 The parabolic fit, also shown in Figure 68 corresponds to the continuous period decrease: P = 45.0068d - 3.64d*10^-4*E (78) +-.0026 +-.12 Figure 68. Upper panel: O-C diagram of SV Vul Lower panel: gamma-velocities for the same Cepheid Figure 69. Delta(O-C) diagram of SV Vul Table 107. gamma-velocities of SV Vul J.D. sigma v gamma sigma n Reference 2400000+ [d] [km/s] [km/s] 25121 249 -1.2 1.4 11 Joy (1937) 32503 181 0.4 0.8 15 Sanford (1956) 43336 63 -2.4 0.3 5 Fernie (1979a) 43502 5 -3.6 1.6 7 Wilson et al. (1989) 43709 75 2.7 1.0 15 Barnes et al. (1987) 44110 67 -2.5 1.2 11 Barnes et al. (1987) 47311 1 4.0 0.6 2 Samus (1990) The deviation of the individual O-C residuals from this parabola, i.e. the Delta (O-C) diagram is shown in Figure 69. Figures 68 and 69 clearly indicate the commencement of a new positive deviation in the mid-eighties. The oscillation about the parabola seems to be cyclic with a cycle-length of slightly longer than ten thousand days (see also Schroder, 1978). The amplitude of this oscillation is considerable: it reaches 0.03 pulsation phase, and by no means is caused by the light-time effect. Since the recent deviation has not been taken into account in calculating the forthcoming maxima, the numerical data for SV Vul in Table 110 may be in error. Regular photometric study of SV Vul is desirable. The gamma-velocity values determined for SV Vul are collected in Table 107. Although the extreme values differ from each other by more than 7 km/s, the variability in the gamma-velocity cannot be stated, because the extreme values have been determined from radial velocity data, either of poor quality or small in number. GENERAL REMARKS A considerable fraction of the Cepheids in this sample has been known to exhibit characteristic period changes. In order to achieve a more complete coverage in the O-C diagram, selected programme stars have been observed with the 50 cm Cassegrain telescope at Piszkesteto Mountain Station of Konkoly Observatory. This telescope is equipped with an integrating photoelectric photometer (containing an EMI 9058QB type multiplier) and standard UBV filters. The individual observational data are listed in Table 108. The mean error of these new photometric measurements is about 0.01 in V and B, and somewhat larger in U. The observations have been transformed into the standard UBV system, but the data on V636 Cas are only differential magnitudes with respect to the comparison star BD +62 259. Table 108. New photoelectric observations of selected Cepheids J.D.Hel. V B-V U-B J.D.Hel. V B-V U-B 2440000+ 2440000+ FF Aql (BY Cas) 3287.491 5.45 0.79 3304.438 5.56 0.78 4162.493 10.24 1.19 3337.388 5.29 0.69 4166.325 10.43 1.28 3382.375 5.22 0.68 4167.490 10.61 1.30 3385.336 5.51 0.83 4203.302 10.54 1.19 3386.312 5.36 0.74 4251.285 10.62 1.30 3388.350 5.41 0.78 3401.363 5.33 0.73 J.D.Hel. V B-V U-B 3438.255 5.50 0.81 2440000+ 3599.595 5.52 0.84 DD Cas 3647.441 5.39 0.75 4874.397 10.26: 1.35: 3737.333 5.44 0.80 4972.244 10.09 1.34 3739.338 5.39 0.74 5229.432 10.25 1.30 3743.424 5.47 0.80 5259.352 10.15 1.30 3765.345 5.51 0.82 5294.352 9.91 1.21 3772.280 5.32 0.71 3789.305 5.20 0.65 J.D.Hel. Delta V Delta(B-V) 3800.218 5.49 0.80 2440000+ 6597.372 5.23 0.66 0.46 V636 Cas 6612.424 5.50 0.80 0.47 5200.515 -0.322 0.832 6614.374 5.36 0.72 0.50 5229.408 -0.293 0.800 6652.382 5.45 0.79 0.64 5259.464 -0.311 0.823 6653.339 5.54 0.80 0.52 7791.437 -0.294 0.896 J.D.Hel. V B-V U-B J.D.Hel. V B-V U-B 2440000+ 2440000+ RW Cam IR Cep 6466.268 8.32 1.21 0.90 7316.500 7.80 0.82 0.48 6467.275 8.16 1.19 0.95 7387.529 7.65 0.75 0.51 6489.364 8.67 1.48 1.02 7388.462 7.85 0.86 0.55 6490.360 8.76 1.49 0.93 7438.233 7.65: 0.71: 0.46: 7174.467 8.28 1.29 0.95 7438.385 7.62 0.73 0.52 7175.473 8.35 1.39 0.98 7439.360 7.90 0.88 0.56 7443.581 8.84 1.51 0.92 7439.439 7.91 0.89 0.53 7444.530 8.93 1.49 0.87 7440.268 7.71 0.78 0.49 7441.236 7.82 0.84 0.55 J.D.Hel. V B-V U-B 2440000+ J.D.Hel. V B-V U-B BY Cas 2440000+ 3382.524 10.13 1.23 SU Cyg 3420.472 10.36 1.21 3304.451 7.15 0.69 3425.456 10.48 1.36 3337.375 6.73 0.58 3426.433 10.48 1.33 3351.527 6.65 0.49 3437.474 10.23 1.12 3363.513 6.45 0.42 3489.328 10.26 1.30 3375.464 6.66 0.48 3490.319 10.59 1.37 3388.415 7.03 0.68 3572.247 10.24 1.25 3401.387 6.84 0.54 4108.530 10.51: 1.20: 3403.325 6.98 0.65 4143.422 10.29 1.21 3424.256 7.05 0.61 4157.406 10.62 1.31 3440.234 6.46 0.41 4159.392 10.37 1.10 4372.461 7.01 0.63 Table 108. (cont.) J.D.Hel. V B-V U-B J.D.Hel. V B-V U-B 2440000+ 2440000+ (SU Cyg) (DT Cyg) 4402.459 6.73 0.55 4159.271 5.66 0.42 4455.393 6.45 0.41 4166.277 5.80 0.50 4458.535 7.16 0.63 4215.191 5.83 0.50 4811.391 7.15 0.67 4458.423 5.92 0.51 4822.436 7.01 0.63 4486.328 5.76 0.44 4840.386 6.54 0.46 4811.473 5.71 0.42 4862.346 7.18 0.61 4822.468 5.86 0.51 4870.303 6.96 0.58 4840.417 5.93 0.55 4874.308 6.75 0.51 5200.451 7.19 0.70 J.D.Hel. V B-V U-B 5224.376 6.59 0.44 2440000+ 5229.331 6.76 0.55 V532 Cyg 5259.286 6.46 0.40 3353.552 9.23 1.14 5294.273 6.58 0.47 3375.562 9.08 1.12 6222.490 7.06 0.70 0.46 3382.460 9.18 1.14 6266.455 6.70 0.52 0.44 3388.498 9.05 1.09 6268.445 7.02 0.69 0.45 3401.451 8.96 1.06 6271.433 6.74 0.56 0.45 3403.349 9.11 1.08 6387.241 6.90 0.64 0.45 3437.297 8.92 0.99 6597.429 6.46 0.40 0.47 3440.262 8.94 0.99 6652.413 6.84 0.63 0.49 3476.218 8.88 1.01 6653.372 7.11 0.70 0.48 3481.227 9.25 1.14 7791.361 7.05 0.69 0.43 3489.218 8.97 1.02 7792.321 7.19 0.71 0.44 4108.486 9.30 1.19 4111.589 9.23 1.18 J.D.Hel. V B-V U-B 4113.401 8.91 1.06 2440000+ 4157.350 9.15 1.15 DT Cyg 4162.418 9.02 1.07 3351.538 5.83 0.48 4166.338 8.94 1.06 3363.538 5.92 0.55 4173.267 9.05 1.10 3375.426 5.82 0.49 4203.267 9.30: 1.05: 3382.359 5.68 0.45 4811.503 9.25 1.15 3382.477 5.70 0.44 4822.498 8.90 1.05 3386.349 5.90 0.53 4840.439 9.23 1.14 3388.452 5.93 0.55 4862.391 9.02 1.07 3403.312 5.94 0.56 4870.347 9.27 1.17 3437.229 5.69 0.44 4874.347 9.14 1.16 3440.248 5.75 0.49 5224.403 9.19 1.19 3489.228 5.74 0.46 5229.359 8.94 1.08 3714.543 5.72 0.38 5259.316 8.93 1.06 3722.566 5.78 0.47 5294.318 9.22 1.12 3736.444 5.83 0.49 6268.511 9.14 1.12 3743.463 5.90 0.54 6387.309 9.26 1.15 3765.357 5.86 0.54 6652.496 9.10 1.13 3772.433 5.73 0.47 6653.475 9.24 1.17 3789.372 5.63 0.44 7388.421 9.20 1.15 3798.260 5.98 0.54 7438.346 9.25 1.14 4108.449 5.91 0.52 7439.405 8.93 1.01 4111.473 5.73 0.47 7443.435 8.99 1.04 4129.387 5.67 0.42 7444.328 9.21 1.17 4157.326 5.75 0.47 7791.339 8.94 1.04 7792.353 9.20 1.17 Table 108. (cont.) J.D.Hel. V B-V U-B J.D.Hel. V B-V U-B 2440000+ 2440000+ TX Del AW Per 4458.468 8.98 0.62 4633.429 7.86 1.22 4462.399 9.33 0.82 4637.427 7.42 1.14 4661.337 7.07 0.93 J.D.Hel: V B-V U-B 6466.321 7.41 1.09 0.73 2440000+ 6467.326 7.56 1.15 0.75 Y Lac 6489.403 7.63 1.09 0.68 4458.516 8.99 0.68 6490.400 7.06 0.89 0.68 7174.389 7.73 1.13 0.71 J.D.Hel. V B-V U-B 7175.407 7.06 0.87 0.71 2440000+ 7443.568 7.64 1.18 0.79 CV Mon 7444.574 7.80 1.21 0.78 6466.360 10.54 1.46 6467.353 10.45 1.37 J.D.Hel. V B-V U-B 2440000+ J.D.Hel. V B-V U-B S Sge 2440000+ 4811.440 5.65 0.86 RS Ori 6597.447 5.74 0.93 0.78 4633.470 8.69 1.14 6614.423 5.82 0.91 0.84 4637.453 8.11 0.87 6652.399 5.32 0.61 0.53 4661.392 8.23 0.96 6653.356 5.38 0.69 0.53 6691.254 5.98 0.98 0.83 J.D.Hel. V B-V U-B 2440000+ J.D.Hel. V B-V U-B SV Per 2440000+ 4633.448 8.99 1.19 SZ Tau 4661.358 9.04 1.02 4633.409 6.66 0.90 4661.261 6.52 0.84 J.D.Hel. V B-V U-B 2440000+ J.D.Hel. V B-V U-B VX Per 2440000+ 4638.304 9.07 1.18 S Vul 4661.299 9.11 1.13 4811.458 9.44 1.73 4822.451 9.14 1.56 J.D.Hel. V B-V U-B 4840.400 8.96 1.57 2440000+ 4870.318 9.31 1.83 AS Per 4874.327 9.40 1.77 3385.570 9.33 1.45 3420.592 9.47 1.44 J.D.Hel. V B-V U-B 3423.588 9.93 1.61 2440000+ 3424.576 9.12 1.28 SV Vul 3437.460 9.85 1.67 4455.426 7.32 1.70 3458.651 9.70 1.56 4458.550 7.41 1.72 3481.315 9.72 1.56 3490.492 9.49 1.52 3546.247 9.79 1.55 3560.415 9.54 1.58 3572.261 9.98 1.72 3598.293 9.28 1.36 Table 109. New radial velocity measurements of selected Cepheids Cepheid J.D.Hel. radial sigma number of dispersion 2440000+ velocity (km/s) lines (A/mm) FF Aql 6727.269 -23.8 km/s 1.6 17 9 DT Cyg 5270.297 1.9 2.3 17 9 AW Per 6726.512 9.7 3.9 21 18 AW Per 6727.498 19.7 2.9 18 18 S Sge 6726.281 -12.9 2.4 22 9 Several spectrograms of binary Cepheids were also taken with the coudee spectrograph attached to the 2m telescope of Rozhen Observatory (Bulgaria). The spectra were obtained at a dispersion of either 9 A/mm or 18 A/mm at H gamma. The individual radial velocity values are listed in Table 109. A brief summary on the period, period changes and gamma-velocity variations is compiled in Table 110. The subsequent columns of this table contain the following data: 1. Name of the Cepheid, 2. Moment of the normal maximum just following J.D. 2450000, 3. Pulsation period expected at J.D. 2450000. This value is calculated with the help of the corresponding (linear or parabolic) ephemeris published in the previous discussion on the given star, 4. Characteristic features in the O-C diagram, 5. Variability in the gamma-velocity, 6. Value of the orbital period, 7. Reference to the paper where the value cited in the previous column has been published. The use of J.D. 2450000 seems to be a reasonable compromise, because the extrapolation of the current elements for 1995 is not hazardous in the overwhelming majority of the cases. There are only eight stars (V572 Aql, BY Cas, DT Cyg, DX Gem, T Mon, VX Per, V440 Per, and SV Vul) in this sample, exhibiting either a quite recent period change or a deviation from the parabolic fit, that may cause some uncertainty in the prediction of the ephemeris. The extension of the O-C diagrams to a larger time-base and the use of more accurate observations resulted in a different from the previous interpretation of the period changes in a number of cases. Instead of a single sudden period change (i.e. two linear sections in the O-C diagram) a continuous change in the period (i.e. parabolic O-C graph) is suggested for VZ Cyg, W Gem, RZ Gem, RR Lac, S Sge, and SW Tau. The phase jump interpretation proposed in Papers I-III has been confirmed in most cases, and in addition, this phenomenon seems to have happened to some more Cepheids. The list of the northern Cepheids showing a phase jump in their pulsation is as follows: FF Aql, BY Cas, DD Cas, DL Cas, X Cyg, SU Cyg, SZ Cyg, DT Cyg, V532 Cyg, V924 Cyg, TX Del, DX Gem, X Lac, CV Mon, RS Ori, SV Per, SZ Tau, T Vul, X Vul. The phase jump appears most clearly in the case of FF Aql and SU Cyg, while its occurrence has to be confirmed in the case of some stars listed above (see Table 110). For FF Aql the phase shift has also been detected in the radial velocity data (Evans et al. 1990b). SU Cyg is unique in the respect that the phase jump was accompanied with a noticeable change in the shape of the light curve. The phenomenon of the phase shift is a feature occurring in binary Cepheids, and its repeated occurrence in the same star (Y Oph in Paper IV, TX Del and T Vul in this paper) suggests that the jump is triggered by the orbital motion. The orbital motion itself can be followed in the O-C diagram via the light-time effect. This phenomenon is best seen in AW Per, but may also be present in the O-C plot of FM Aql, RW Cam, Y Lac, and RS Ori. This latter Cepheid is especially important because RS Ori might be the first case of exhibiting a phase jump and the light-time effect in the same O-C diagram. The alternating period changes on a time-scale of several years or decades can be attributed to solar type activity cycles (Hall, 1990). The present sample contains several Cepheids showing quasi-cyclic changes in the pulsation period, viz. Zeta Gem, T Mon, S Vul, and SV Vul. Although the number of Cepheids forming this group is small, it may be significant that the shortest pulsation period for a member is longer than 10 days. A problem to be solved in the future is the differentiation between the phase slip (a slower or more gradual phase jump) and the alternate period changes due to magnetic effects. The solar type activity cycle does not necessitate the duplicity of the star involved, although a companion star usually strengthens the activity. The analysis of the radial velocity observations has resulted in revealing a number of new spectroscopic binary Cepheids or candidates. The most promising cases are: KL Aql, Eta Aql, SU Cas, V636 Cas, BZ Cyg, MW Cyg, V386 Cyg, W Gem, RZ Gem, AD Gem, RS Ori, SV Per, SW Tau, T Vul, U Vul. An attempt was made to find the value of the orbital period for Cepheids with ample radial velocity data, but each spectroscopic period derived here has to be confirmed with the help of additional measurements. Table 110. Summary on the periods, period changes and duplicity Cepheid Norm.max. Period O-C graph v gamma P orb Source 2400000+ [d] SZ Aql 50012.262 17.141745 parabolic (+) ? TT Aql 50000.687 13.754954 linear var.? FF Aql 50004.063 4.470936 lin. with PHJ var. 1430 Evans et al. (1990) FM Aql 50003.142 6.114265 linear (+LTE?) const.? 2800 ? this paper KL Aql 50002.539 6.108015 linear var. short this paper V572 Aql 50000.287 3.768001 linear (?) - V1344 Aql 50000.187 7.476787 linear var.? Eta Aql 50000.175 7.176758 parabolic (+) var. 926 ? this paper RT Aur 50000.279 3.728198 linear const. AN Aur 50003.628 10.289563 linear var. RW Cam 50012.740 16.415015 lin. with LTE or PHJ var. 7000 ? this paper SU Cas 50000.720 1.949325 linear var. 462.5 ? this paper SZ Cas 50009.120 13.645356 parabolic (+) var.? BY Cas 50000.908 3.222199 lin. with PHJ ? DD Cas 50001.805 9.811656 lin.(+ PHJ?) var.? DL Cas 50004.712 8.000598 lin. with PHJ var. 688.0 Harris et al. (1987) IX Cas 50003.203 9.154549 irregular var. 110.29 Harris and Welch (1989) V636 Cas 50005.366 8.375735 linear var. IR Cep 50000.719 2.114088 linear (+EPCH) one V351 Cep 50000.179 2.806052 linear (+EPCH) - X Cyg 50007.657 16.385692 lin. with PHJ var.? SU Cyg 50000.682 3.845512 lin. with PHJ var. 549.16 Evans (1988) SZ Cyg 50000.868 15.110228 lin. with PHJ var. TX Cyg 50003.317 14.711635 linear (+EPCH) var.? VZ Cyg 50004.454 4.864372 parabolic (-) var.? BZ Cyg 50001.523 10.142222 linear var. DT Cyg 50002.270 2.499086 lin. with PHJ var. MW Cyg 50004.005 5.954666 linear var. V386 Cyg 50001.131 5.257635 linear var. V532 Cyg 50000.792 3.283494 lin. with PHJ var.? V924 Cyg 50002.268 5.571305 lin. with PHJ - V1334 Cyg 50002.920 3.332804 linear var. <1240 this paper V1726 Cyg 50003.570 4.236978 one TX Del 50000.932 6.165904 lin.(+ PHJ?) var. 133.15 Harris and Welch (1989) Table 110. (cont.) Norm.max. Period O-C graph v gamma P orb Source Cepheid 2400000+ [d] [d] W Gem 50003.897 7.913277 parabolic (-) var. RZ Gem 50002.248 5.528942 parabolic (-) var. 886? this paper AD Gem 50002.061 3.787990 linear var. DX Gem 50001.290 3.136779 lin. with PHJ var.? Burki (1985) Zeta Gem 50007.231 10.149414 parabolic (-) var.? V Lac 50000.245 4.983002 parabolic (-) var.? X Lac 50000.890 5.444322 lin. with PHJ var.? Y Lac 50000.797 4.323769 linear var.? Z Lac 50000.760 10.885600 parabolic (-) var. short this paper RR Lac 50001.448 6.416335 parabolic (+) var. BG Lac 50004.552 5.331902 linear var.? T Mon 50025.601 27.020097 lin.On par.(+) var. very long Gieren (1989b) SV Mon 50009.142 15.232582 linear const. CV Mon 50002.177 5.378804 lin. with PHJ one V465 Mon 50000.505 2.713006 linear (+EPCH) var. Burki (1985) RS Ori 50001.701 7.566841 lin. with LTE and PHJ var. long ? this paper GQ Ori 50001.590 8.616283 linear one SV Per 50004.946 11.129319 lin.(+ PHJ?) var. VX Per 50008.306 10.886972 linear (+EPCH) const. AS Per 50003.220 4.972540 linear one AW Per 50000.041 6.463622 lin. with LTE var. 13100 Welch and Evans (1989) V440 Per 50003.336 7.572498 linear (?) const. S Sge 50004.834 8.382184 parabolic (+) var. 676.2 Slovak and Barnes (1987) SW Tau 50001.443 1.583540 parabolic (-) var. SZ Tau 50001.329 3.149138 lin. with PHJ var.? S Vul 50038.692 68.500 lin. sections ? T Vul 50003.860 4.435453 lin. with PHJ var. 1745 this paper U Vul 50005.698 7.990821 linear (+EPCH) var. 868 ? thiis paper X Vul 50002.860 6.319490 lin.(+ PHJ?) var. SV Vul 50013.768 44.9558 parabolic (-) const.? Legend: LTE : light-time effect (+) and (-) : continuous period increase or decrease PHJ : phase jump(s) ? : poorly covered radial velocity curve, EPCH : earlier period change(s) the gamma-velocity cannot be determined one : only one series of radial velocity data - : radial velocity observations have not been made A new method, independent from the previously used ones, has been proposed for revealing the presence of a companion. This evidence is based on the amplitude ratio of the light and radial velocity variability (see V440 Per). The new discoveries and uncertainties confirm the fact that there is a huge number of Cepheid variables deserving regular attention (photometric and radial velocity studies), and/or occasional close inspection in the form of a detailed spectroscopic analysis. The author is grateful to Drs. M. Kun and B. Szeidl for their useful comments and encouragement during accomplishing this project. Thanks are also due to the staff of Rozhen Observatory (especially to Dr. I. Iliev) for the help in obtaining and reducing the new radial velocity data listed in Table 109. Generosity of several colleagues (putting at my disposal their observational data prior to publication) is also acknowledged: Dr. N.N. Samus handed me the whole list of his group's radial velocity results, Dr. G. Wallerstein sent a list of photometric observations on SW Tau, and Dr. R.F. Griffin informed me about the unpublished radial velocity data on AW Per. The author's thanks are also extended to Mr. P. 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