COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 3151



                                     Konkoly Observatory
                                     Budapest
                                     26 February 1988

                                     HU ISSN 0374-0676



           THE ELLIPSOIDAL VARIABILITY OF 75 PEGASI

   75 Pegasi = HD222133 = HR8963 is a single-lined spectroscopic binary
with the very short period of 0.5021035 day, and an extreme value of mass
ratio (Hube and Gulliver, 1985). Analysis of the spectroscopic data led to
the conclusion that the primary component fills, or is close to filling, its
critical Roche lobe. Furthermore, it was predicted that a shallow primary
eclipse (~0.02 m) would occur. After the fact, it is clear that we ought to
have made explicitly a prediction of ellipsoidal photometric variations.
   75 Pegasi has been observed since September 1987 with the 2-star
photometer on the 0.5m telescope of the Devon Astronomical Observatory.
Standard B and V filters have been employed. On all but the first night, the
comparison star in channel-2 has been SAO108725 and the check star was
BD +17 4953. The latter two stars have been constant relative to one another
to better than about 0.03. Corrections for atmospheric extinction are
negligible with our observing procedures. Corrections for gain variations
between the two channels have been made. We have not made transformations
from the local to the standard UBV system.
   On September 23/24, 1987, a complete sinusoidal light variation was
detected during 7 hours of continuous observing, and with phasing relative
to the spectroscopic orbit consistent with ellipsoidal variability. 
Observations on subsequent nights have confirmed the photometric variability and
the initial interpretation of it. In this note we present a sample of the
available photometric data and results of a very preliminary analysis.
   In Figure 1 we have plotted the magnitude differences B(VAR) - B(COMP)
obtained on five nights and averaged in phase intervals of 0.025P. Phases
are from T = HJD2442644.131 as found in the spectroscopic orbital solution.
The photometric data are consistent with the spectroscopic period of
0.5021035 day.


                              [FIGURE 1]


   There is some evidence - not revealed in Figure 1 because we have
averaged together data from several nights - for cycle-to-cycle variations in
the shape of the light curve. These might be due to irregular mass flow from
the Roche-lobe-filling primary, or to some intrinsic instability in one of
the components. At this time, we cannot exclude observational or reduction
errors.

   Within the observational uncertainties, the two maxima are equal. The
deeper minimum at primary conjunction may include contribution from the
predicted shallow eclipse, but that will not be known with certainty until a
detailed analysis has been completed. We have used a least-squares routine
to fit the following Fourier series to the data in Figure 1


        DeltaB = -4.549 + 0.0029 cos Theta - 0.0431 cos 2Theta +
                (+-0.001) (+-0.0018)         (+-0.0019)

                 + 0.0162 sin Theta - 0.0043 sin 2Theta
                   (+-0.0019)         (+-0.0018)

where the uncertainties are standard deviations. The predominance of the
cos2theta term is consistent with a gravitationally distorted primary component
and ellipsoidal light variation.

   A complete discussion of this binary system will be presented elsewhere
after the photometric data have been transformed to the standard UBV system
and analysed in conjunction with the spectroscopic data.



                     DOUGLAS P. HUBE and BRIAN E. MARTIN

                     Department of Physics
                     University of Alberta
                     Edmonton, Alberta
                     Canada T6G 2J1


                         and

                     AUSTIN F. GULLIVER

                     Department of Physics and Astronomy
                     Brandon University
                     Brandon, Manitoba
                     Canada R7A 6A9


Reference:
Hube, D.P. and Gulliver, A.F., 1985, Publ. Astron. Soc. Pacific 97, 280. [BIBCODE 1985PASP...97..280H ]