COMMISSIONS 27 AND 42 OF THE IAU
               INFORMATION BULLETIN ON VARIABLE STARS
                            Number 4093

                                                    Konkoly Observatory
                                                    Budapest
                                                    28 September 1994                      
                                                    HU ISSN 0374 - 0676




             OPTICAL VARIABILITY IN SAO 20517 AND ITS
            POSSIBLE IDENTIFICATION AS AN X-RAY SOURCE



During a recent campaign to observe the massive eclipsing binary system
HD219634 the field star SAO20517 (alpha(2000)=23h15m30s, 
delta(2000)=61d52') was used as one of 3 comparison stars. These
observations reveal that SAO20517 is variable with a variation of about 0.06
mag in the V passband. Figure 1 illustrates the photometric variation with 
respect to SAO20532 during a two month period from July 1994 through August
1994. Figure 2 shows the same data phased with a period of 15.864d. The
star SAO20532 (alpha(2000)=23h16m48s, delta(2000)=61d48')
has been used as the principal comparison star and SAO20526 
(alpha(2000)=23h16m42s, delta(2000)=61d41') as a
check star in these observations. Data have thus far been obtained using The
King's University College Observatory CCD photometer on a 0.2 m telescope and 
the University of Alberta, Devon Observatory two-channel photometer attached
to the 0.5 m telescope. The data, as presented have not been transformed to the
standard UBV system.

SAO20517 may be an x-ray source. It can be tentatively identified with the
final source listed by Helfand and Caillault (1982), and coincides with a weak,
unresolved source on plate 7-434 of "The Eistein Observatory Catalog of IPC X-Ray
Sources". Bidelman (1994, private communicatio) suggests a spectral type of K0IV
based on examination of a contaminated spectrum on an objective prism plate.
This, and the light curve shown in Figure 2 lead us to suggest that SAO20517 
is either an RS CVn or ellipsoidal binary system. The relatively high incidence
of transient x-ray source at high galactic latitude (Garcia et al., 1980) lends
credence to the suggestion that SAO20517 is an RS CVn system. 



                              [FIGURE 1]

Figure 1. V filter light curve for SAO20517 covering a two month span. The solid curve represents
a simple sinusoidal fit to data. Observations made on TKUC CCD and 0.2m reflector.



                              [FIGURE 2]


Figure 2. Light curve of observation shown in Figure 1 phased with a period of 15.864
days. Solid line shows the fit provided by the sum of the first four Fourier components.

At the same
time evidence of ellipsoidal variation is provided by a least-squares fit of
the first 4 Fourier components to the light curve. The variation can be described
via:
Delta v=0.70450.0043sin(Theta)+0.0064sin(2Theta)+0.0064cos(Theta)+0.0251cos(2Theta)
when v is represented as a function of phase (Theta). The
standard error in the Fourier coefficients is on the order of 0.002 mag. The
residual error in the Fourier fit is approximately 0.007 mag which is typical of
the scatter in the observations. The dominance of the cos(2Theta) term 
supports the suggestion that SAO20517 is an ellipsoidal binary system.

The authors are obtaining additional multicolour (UBVR) photometric data. 
Spectroscopic data are being obtained Dominion Astrophysical Observatory,
Victoria. A complete discussion of this system will be published elsewhere.
We thank Dr. W. Bidelman for a very informative discussion about this star
and related matters.




B. MARTIN
The King's University College
9125-50th Street
Edmonton, AB.CA.
T6B 2H3
(bmartin @ kingsu.ab.ca)

D.P. HUBE
E. STEINBRING
S. CARTLEDGE
Department of Physics
University of Alberta
Edmonton, AB.CA.
T6G 2J1



References:

Bidelman, W., 1994, private communication

Garcia, M. et al., 1980, Astrophys. J., 240, L107 [BIBCODE 1980ApJ...240L.107G ]

Helfand, D. J. and J.-P. Caillault, 1982, Astrophys. J., 253, 760 [BIBCODE 1982ApJ...253..760H ]