COMPARISON BETWEEN THE VISUAL AND PHOTOGRAPHIC LIGHT 
         CURVES OF SOME RV TAU STARS AND NOTES ON T TAU AND 
                          RW AUR STARS
                   by P. AHNERT, Sonneberg


    It is a well-known fact that pulsating stars of later spectral type show 
a displacement of phase between visually and photographically observed 
minima and maxima.
    RV Tauri-stars show the same phenomenon but owing to their 
frequently disturbed light curves these displacements differ from one another.

 

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In most cases the photographically observed minima precede the visually 
observed ones but in some single cases they might happen at the same time. 
    In the years 1951 to 1953 I found for the limited material:

               t_min_vis-t_min_ph of R Sge=+2.4d+-0.9d
                 "         "       " V Vul=+2.2d+-3.0d

 
 

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    The minima of AC Her follow the same rule, but there are too few well 
observed minima, so I could not derive an exact value of the displacement.
    The maxima of each star show the same tendency, although they are 
mostly flat and not exactly to define.
    In an investigation by Shdanova and Zessevich the mean photographic 
and visual light curves of some RV Tau-stars have been compared. I may 
direct your attention to the individual course of both curves: The course 
can be very different, especially the maxima and the secondary minima can 
appear in a very different shape. Therefore, also the course of the colour-index
must be rather irregular, and not at all an accurate function of the 
phase or the brightness of the star. Only in general can be said that the  
colour-index is larger at the time of the minima than of the maxima.
    Accidentally in these short series of observations the change of primary 
and secondary minima takes place twice: With R Sge and V Vul. In each 
case the change is introduced by a flattening of the primary minima till both 
kinds of them are nearly of the same depth. After some cycles the former 
secondary minima become deeper until the characteristic shape of the RV Tau 
light curve is reestablished.
     Now I may show you some photographic light curves of T Tauri and 
RW Aurigae stars. I have selected stars of different spectral type, hoping 
that any correlation might be found between the spectral class and the type 
of light variation. But I must confess that no relation can be stated.
     I distinguish two species of these stars, only with regard to their 
photometric characteristics
 (I) Rapid, totally irregular changes of light with large amplitude (2 to 4 
     magn.)
     (a) without standstills (prototype RW Aur)
     (b) mostly bright with small, slow fluctuations and occasional short, 
         deep, and irregular minima (WW Vul)
(II) Mostly slow changes of light with small amplitude (< 1.0m, similar mu Cep),
     interrupted by more rapid fluctuations also of small amplitude (< 1.0m) 
     (prototype T Tau).
I quote for these types the following stars:

I a  V Sge  (Ob)              II CT Tau  (A0)
     RR Tau (A2e II-III)         UX Ori  (A2e)
     CQ Tau (F5)                 BN Ori  (A7)
     RW Aur (dG5)                RY Tau  (dG6e)
I b  WW Vul (A2)                 T  Tau  (dG5e)
                        peculiar EM Cyg  (?)

     V Sge is an old Nova, not an RW Aur-star but photometrically it resembles 
mostly the prototype RW Aur.
     RR Tau seemed to be a real member of the RW Aur-group - in relation 
to its light curve - but now the new spectral classification makes this  
assumption doubtful.
     CQ Tau has nearly the same amplitude as the following
     RW Aur, but its fluctuations seem to be a little slower than the jumps 
of the prototype star. The spectrum F5 is not contradictory to the right RW 
Aur-character.


 

                            Abb. 4. V Sagittae 1950-1955

 

                            Abb. 5. RR Tauri 1951-1956

 

                            Abb. 6. CO Tauri 1950-1956

 

                            Abb. 7. RW Aurigae 1951-1956.


       WW Vul sometimes resembles the stars of group II when its light 
changes are small and slow; but at other times it turn very rapidly faint 
and bright again. After such an algol-like minimum a rapid and irregular 
fluctuation might follow.
       The quickest star of the second group is CT Tau with the earliest 
spectrum A0, whereas all the other members of this group with spectra of 
A and G show only occasionally rapid fluctuations, most of them resemble 
mu Cep.
     EM Cyg, probably an old Nova, is also a remarkable star, photometrically 
between the two groups. Sometimes it shows semi-periodic waves, almost 
similar to mu Cep, sometimes it fluctuates irregularly, and now and then it
flares for somedays.
     At last I have light curves of two U Gem-stars. I may show that sometimes 
other well defined kinds of variable stars can also behave unusually.


 

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     EQ Mon showed during 3000d a period of about fourteen days, so well 
followed, that a mean light curve could be drawn. In the following time the 
star becomes irregular again.
    SS Cyg resembles now and then for a few weeks an RW Aur-star with 
short fluctuations of different amplitudes.


 

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