COMMISSIONS 27 AND 42 OF THE IAU
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3876

                                           Konkoly Observatory
                                           Budapest
                                           4 May 1993

                                           HU ISSN 0374 - 0676


             THE DISCOVERY OF APSIDAL MOTION IN
                THE BINARY SYSTEM alpha CrB


The first photoelectric light curve of the eclipsing binary alpha CrB (P=14.36d,
e=0.37, A0+G6) was obtained by Stebbins (1928). It showed a shallow (~0.1m)
primary minimum. The secondary minimum was detected by Kron and Gordon (1953;
hereafter referred to as KG) in the near infrared band lambda_eff = 7230
angstrom. The depth of this minimum is only 0.02m. Koch (1973) has shown that
the star must have appreciable apsidal motion, mostly relativistic.
Ebbighausen (1976) tried to detect this effect, but his attempt failed due to
insufficient accuracy of the spectrographic determination of omega. We have
estimated the theoretical value of the periastron advance using equations from
Sterne (1939) and Rudkjobing (1959) and the parameters of the system from
Tomkin and Popper (1986). The relativistic part of the periastron advance is
omega-dot_rel= 0.0046d/year and the part due to the tidal and rotational
distortion of the stars supposing synchronous rotation of the components is
omega-dot_class = 0.0014d/year. The sum of these values corresponds to a time
shift of the secondary minimum by nearly 17 minutes in 40 years that elapsed
from the epoch of KG observations in 1946-1948. This value seems quite
measurable and does not differ from Koch's result. But taking into
consideration the high rotational velocity of the primary component v sin i=110
km/sec (Slettebak et al., 1975) and supposing i=90d one can obtain
omega-dot_class+omega-dot_rel~=0.0206d/year - even three times greater than Koch's
value.

Our observations were carried out with the pulse counting photometer equipped
with an EMI 9863 (S-20 cathode) and 48 cm reflector in the mountain station of
Moscow University near Alma-Ata (altitude 3000 m). To reduce the flux from
this bright star (V=2.23m) and to make the atmospheric correction easier, we
used two narrow interference filters centered on lambda_1,2 = 4600, 7500
angstrom (compare with Stebbins lambda_eff = 4600 angstrom and KG). Most of
the observations were made in the 7500 angstrom filter. We have used HD 135502
="C" as prime standard and HD 143761 as the check star. These stars proved
to be constant within the probable error of an observation sigma = +-0.005m
during all the years 1986-1992. But alpha CrB itself has shown variability between
minima. Some nights it had delta Sct-like oscillations with a quasi period of
40 minutes and an amplitude near 0.01m. To reduce the influence of this
variability, we have calculated the corrections for every observational night.
Folding the observations of appropriate phases in computer memory we have
obtained the individual times of mean primary and mean secondary minima for all
observational sets including KG and Stebbins data, see Table 1. To obtain the
precise periods for both minima we processed all observations with the
Jurkevich algorithm. The resulting formula for primary minima is:

            JD_hel =2 447 346.1168+17.359 90016d * E
                                           +-13
(Our observations in 7500 angstrom folded with this period are shown in Figure
1.)

for secondary minima:

            JD_hel=2 447 010.3923+17.3599203d * E
                                        +-30

The difference between these periods proves the existence of an apsidal motion
with the period U=46000+-8000 years or omega-dot_obs = 0.0078d+-0.0012d/year.
One can see that the observed value is 2.6 times smaller than the theoretical
one. We cannot explain such discrepancy now and more observations in infrared,
especially after the year 2000 are badly needed.


                           [FIGURE 1]

                  Figure 1. Light curve of Alpha CrB


                           Table 1

              Min I  O-C   Min II          O-C
JDhel 2400000+             JDhel 2400000+              Author
    23163.7754   +0.0015   -               -          Stebbins
          +-10
    32329.8019    0.0000   32410.6976      -0.0017    Kron, Gordon
           +-6                   +-30
    47346.1168    0.0000   47010.3923       0.0000    Volkov
           +-+6                  +-25

I would like to thank Dr. Khaliullin for directing my attention to the problem
and for fruitful discussions.


                         I. M. VOLKOV
                         Sternberg Astronomical
                         Institute, University
                         of Moscow 119899
                         Universitetsky pr., 13

References:

Ebbighausen, E. G., 1976, Publ. Dom. Astrophys. Obs., Victoria, B. C., 14, 411  [BIBCODE 1976PDAO...14..411E ]
Koch, R. H., 1973, Astrophys. J., 183, 275 [BIBCODE 1973ApJ...183..275K ]
Kron, G. E., and Gordon, K. C., 1953, Astrophys. J., 118, 55 (KG) [BIBCODE 1953ApJ...118...55K ]
Rudkjobing, M., 1959, Ann. Astrophys., 22, 111 [BIBCODE 1959AnAp...22..111R ]
Slettebak, A., Collins, G. W., Boyce, P. B., White, N. M., and Parkinson, T.
   D., 1975, Astrophys. J. Suppl., 29, 137 [BIBCODE 1975ApJS...29..137S ]
Stebbins, J., 1928, Publ. Washburn Obs, 15, 41 [BIBCODE 1928PWasO..15...41S ]
Sterne, T. E., 1939, Mon. Not. R. Astron. Soc., 217, 305
Tomkin, J., and Popper, D. M., 1986, Astron. J., 91, 1428 [BIBCODE 1986AJ.....91.1428T ]