COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1509

                                 Konkoly Observatory
                                 Budapest
                                 1978 November 30




         IS THE CEPHEID ZETA GEMINORUM A VISUAL BINARY ?


     The 10.15d classical Cepheid zeta Geminorum has three nearby companions
which together form the system ADS 5742. The closest star to the Cepheid has
remained relatively fixed over the years in position angle 84d at a separation
of about 87 arcsec (Jeffers, van den Bos, and Greeby 1963), and one is naturally
inclined to the possibility that it may be a physical companion to the Cepheid.
For the purpose of investigating this possibility, Fernie (1969) obtained
photoelectric UBV photometry for this star in February 1967. The results are
retabulated below:


                 V = 11.46 +- 0.02
                B-V = 0.45 +- 0.02
                U-B = 0.06 +- 0.02

     If one makes the reasonable assumption that the star is a normal
dwarf, the UBV data permit reddenings of E_B-V = 0.06 +- 0.02 for an F star and
0.55 +- 0.03 for a B star. The former estimate agrees better with the field
reddening found by Feltz and McNamara (1976) for stars near the Cepheid, and
also with the fact that galaxies are easily detected on the POSS plates
containing zeta Gem. Unfortunately, at the time of the original investigation,
neither value appeared to coincide with the expected reddening of E_B-V = 0.15
for the Cepheid (Fernie 1967). In recent years, however, there has been a
substantial revision of the colour excess scale for long period Cepheids, with
the consequence that current estimates of E_B-V for zeta Gem, as derived from its
observed colours by Parsons and Bouw (1971), Parsons and Bell (1975), and Dean
et al. (1978), range from 0.03 to 0.07. This leads us to conclude that ADS 5742
A and B may indeed form a physical system, if the Fainter star is an F dwarf.
     The consequences of this conclusion are of interest. According to the
Cepheid period-age relation of Kippenhahn and Smith (1969), the expected age for
zeta Gem is about 3 x 10^7 years. An F dwarf of the same age should lie on the
zero-age main sequence (ZAMS). Thus, if we make a reasonable estimate of
M_V = +3.46 +- 0.10 for ADS 5742 B based upon ZAMS fitting of the UBV data, we
derive a corresponding intensity-mean value of <M_V> = -4.17 +- 0.10 for zeta Gem.
This value is in excellent agreement with estimates derived from existing
period-luminosity relations or from applications of the surface brightness
technique (Barnes et al. 1977). Also, the corresponding distance of 366 +- 17 pc
to the system results in a projected separation of about 0.15 pc for the two
stars. If they are associated gravitationally, they must have an orbital period
in excess of 10^6 years. Thus, the lack of any evident change in the location
of B with respect to A over the period of past observations is a logical
consequence of (although not evidence for) actual physical association.
     The results are clearly encouraging and worthy of further investigation.
We therefore urge observers with access to moderate-sized telescopes to make
additional observational studies of the zeta Gem system. All stars in the system
could be profitably studied, both by photometric and spectroscopic techniques.


                     D.G. TURNER
                     J.D. FERNIE
                     David Dunlap Observatory
                     Richmond Hill, Ontario
                     Canada



References:

Barnes, T.G., Dominy, J.F., Evans, D.S., Kelton, P.W., Parsons, S.B., and
   Stover, R.J. 1977, Monthly Notices Roy. Astron. Soc. 178, 661. [BIBCODE 1977MNRAS.178..661B ]
Dean, J.F., Warren, P.R., and Cousins, A.W.J. 1978, Monthly Notices Roy. Astron.
  Soc. 183, 569. [BIBCODE 1978MNRAS.183..569D ]
Feltz, K.A., Jr., and McNamara, D.H. 1976, Publ. Astron. Soc. Pacific 88, 699. [BIBCODE 1976PASP...88..699F ]
Fernie, J.D. 1967, Astron. J. 72, 422. [BIBCODE 1967AJ.....72..422F ]
Fernie, J.D. 1969, J. Roy. Astron. Soc. Can. 63, 133. [BIBCODE 1969JRASC..63..133F ]
Jeffers, H.M., van den Bos, W.H., and Greeby, F.M. 1963, Publ. Lick Obs. XXI. [BIBCODE 1963icvd.book.....J ]
Kippenhahn, R., and Smith, L. 1969, Astron. Astrophys. 1, 142. [BIBCODE 1969A&A.....1..142K ]
Parsons, S.B., and Bouw, G.D. 1971, Monthly Notices Roy. Astron. Soc. 152, 133. [BIBCODE 1971MNRAS.152..133P ]
Parsons, S.B., and Bell, R.A. 1975, Dudley Obs. Rept. no. 9, p. 73. [BIBCODE 1975DudOR...9...73P ]