COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2259

                                     Konkoly Observatory
                                     Budapest
                                     1983 January 5

                                     HU ISSN 0374-0676

        DELTA ERIDANI: A VERY BRIGHT NEW VARIABLE STAR


   This is a very bright star (V = 3.5mag) in which Wilson (1963) reported
Ca II H and K emission of intensity 1 on his 1-to-5 scale and which Keenan
and Pitts (1980) recently classified K0+ IV. Examining the catalogue of
Abt and Biggs (1972) and a few others we noted that published radial 
velocity measures seem to indicate a variation somewhat larger than can be 
accounted for by the expected uncertainties. These three facts made us 
suspect delta Eridani might be an RS CVn-type binary and therefore might be
photometrically variable, as are most members of that class. The Fourth
Edition of the Yale Bright Star Catalogue notes delta Eridani as "VAR?"
but with no accompanying bibliographic reference.
   Henry observed delta Eridani photoelectrically on 13 nights with the
No. 4 16-inch at Kitt Peak National Observatory and on 4 nights with the
24-inch at Dyer Observatory; Renner observed on 9 nights with the 10-inch
at Scuppernong Observatory; Fisher observed on 7 nights with his 12.5-inch
in San Antonio, Texas; and Landis observed on 6 nights with his 8-inch in
Locust Grove, Georgia. All observers used epsilon Eridani as the comparison 
star, obtained 2 or 3 differential measures on each night, corrected for
extinction, and transformed to V of the UBV system.
   Examination of our photometry showed that delta Eridani is variable
with an amplitude of about 0.02mag and a period of about 10 days. The figure
below is a plot of nightly means of the DeltaV values from Kitt Peak, where Delta
is in the sense variable minus comparison. A bit more than one complete
cycle seems to be defined. The other observations, spread out over longer
intervals of time and/or somewhat lower in accuracy, do not define this
small-amplitude variation as well but are consistent with a total range of
about 0.02mag.


                           [FIGURE 1]


   In the RS CVn binaries the period of any slow light variation (not 
attributable to eclipse, ellipticity, or reflection) is taken to be a measure
of the rotation period of one star presumed to be darkened unevenly with
starspots. Moreover, in all known RS CVn binaries except one (lambda Andromedae) 
the rotation is synchronous with the orbital motion to within a few
percent (Hall 1981). Therefore we argue that, if delta Eridani indeed is a
binary system, the orbital period should be around 10 days also. Spectroscopic
observers will enjoy obtaining spectrograms of this bright star to
check our prediction. The total radial velocity variation might, however,
be quite small because of the small orbital inclination. A K0 subgiant with
a radius of 5 RSun and a rotation period of 10 days would have an equatorial
velocity of 25 km/sec. The value of V sin i = 2.2 km/sec measured by Smith
(1979) therefore would imply an orbital inclination of only i = 5deg. An 
inclination this small could, we point out, help explain why the light 
variation in delta Eridani is so small in amplitude.
   If delta Eridani proves to be a binary, it will be the second brightest
RS CVn binary known, with only alpha Aurigae being brighter.



    D.S.H. acknowledges support from N.A.S.A. Research Grant NSG 7543, and
H.J.L. acknowledges his membership in the A.A.V.S.O.




              GORDON F. FISHER

              2319 Gramercy Place
              San Antonio, Texas 78201

              DOUGLAS S. HALL /a
              GREGORY W. HENRY /ab

              Dyer Observatory
              Vanderbilt University
              Nashville, Tennessee 37235

              HOWARD J. LANDIS

              50 Price Road West
              Locust Grove, Georgia 30248

              THOMAS R. RENNER

              Scuppernong Observatory
              4512 Deerpark Drive
              Dousman, Wisconsin 53118

              STEVEN N. SHORE

              Warner and Swasey Observatory
              Case Western Reserve University
              East Cleveland, Ohio 44106




References:


Abt, H. A. and Biggs, E. S. 1972, Bibliography of Stellar Radial Velocities. [BIBCODE 1972bsrv.book.....A ]
Keenan, P. C. and Pitts, R. E. 1980, Ap. J. Suppl. 42, 541. [BIBCODE 1980ApJS...42..541K ]
Hall, D. S. 1981, in Solar Phenomena in Stars and Stellar Systems, edited by
  R. M. Bonnet and A. K. Dupree, p. 431. [BIBCODE 1981spss.conf..431H ]
Smith, M. A. 1979, P.A.S.P. 91, 737. [BIBCODE 1979PASP...91..737S ]
Wilson, O. C. 1963, Ap. J. 138, 832. [BIBCODE 1963ApJ...138..832W ]





a) Guest Investigator, Kitt Peak National Observatory, operated by the
  Association of Universities for Research in Astronomy, under contract with
  the National Science Foundation.
b) now at the McDonald Observatory, Fort Davis, Texas.