COMMISSION 27 OF THE I. A. U.
                INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3535

                                                      Konkoly Observatory
                                                      Budapest
                                                      31 October 1990

                                                      HU ISSN 0374 - 0676


                  1990 BVR PHOTOMETRY OF XY UMa

   The close binary XY UMa (= +55d 1317, SAO 27143) is No. 69 in the
catalogue of chromospherically active binaries of Strassmeier et al. (1988).
Geyer has made synoptic optical observations since 1955. From coordinated
x-ray and optical observations, Bedford et al. (1990) concluded that the G
primary star emits the soft x-ray flux, which showed a clear primary eclipse.
The coronal plasma has an enhanced component on the side facing the primary
star; a photospheric active region underlies a similar span in longitude. With
the launch of ROSAT, new x-ray observations are possible for XY UMa.
Consequently, we have decided to continue our optical photometry at Capilla
Peak Observatory (CPO).
   Our new observations were done on the nights of March 3 and 4 and April
16, 1990 UT. We followed a similar procedure as for our 1988 data (Zeilik et
al., 1988), except that we have a new filter set (Beckert and Newberry, 1989)
for our CCD system (Laubscher et al., 1988). The effective wavelengths are: B,
438.5 nm; V, 559.4 nm; and R, 680.4 nm. We used Geyer's comparison star (SAO
27139). The observations were reduced with a software mask with a diameter of
20 arcsec. Exposures were chosen so that S/N > 100. Phases were calculated
from the ephemeris in Strassmeier et al. (1988).
   Figures 1 through 3 give the B, V and R delta magnitudes in the instrumental
system. In Figure 4, we compare the observations (open circles) to an
eclipsing binary model fit (solid line), following the procedures of Budding
and Zeilik (1987); the units are normalized intensity. The difference between
these two curves defines the distortion wave for the system at this epoch. We
then applied a circular, black (T=0) spot model to the distortion wave. We
found that the data were not precise enough to extract a determinate solution
for latitude in a four-parameter fit. Therefore, once we found a preliminary
longitude value, we fixed this parameter and searched Chi-square solution
hyperspace for a latitude value. We found a value of near 45 degrees at V
band. We then fixed the latitude at 45 deg and found the following spot
parameters:

Wavelength        Longitude      Radius         Chi-square

       B          113d +- 5.2d    15.9d +- 0.7d     93.8
       V          108d +- 14d     8.9d  +- 1.0d    182.4
       R          126d +- 7.4d    12.2d +- 0.6d    110.5

   We attempted to use the V and R observations to calculate a spot
temperature; however, the overall errors (= 0.04 intensity units) were too
large to do so. After our fits with one spotted region, we "cleaned" the
original light curves of the maculation effects and attempted to find another
active region. We were unable to find another one.


                              [FIGURE 1]
                              [FIGURE 2]
                              [FIGURE 3]
                              [FIGURE 4]


   In our 1987 data (Heckert and Zeilik, 1988), we found one spotted region at
a longitude = 269 deg +- 6 deg, a latitude of 40 deg +- 36 deg, and a radius of
9.2 deg +- 4.0 deg at V band. These fell into the active longitude belt near the
270 deg quadrature longitude. In 1990, we now have the active region in the
active longitude belt near 90 deg, which upholds the trends for the
short-period systems. Their primary activity signature involves a switching of
activity between the two active longitude belts.


                            MICHAEL ZEILIK, CECILIA DE BLASI,
                            SHAWN GORDON and MICHAEL LEDLOW

                            Capilla Peak Observatory
                            Institute for Astrophysics
                            University of New Mexico
                            Albuquerque, New Mexico 87131 USA



REFERENCES:

Beckert, D.C. and Newberry, M.V. Pub. Astron Soc. Pacific, 101, 849-858 (1989).  [BIBCODE 1989PASP..101..849B ]
Bedford, D.K., Jeffries, R.D., Geyer, E.H., and Vilhu, O., Mon Nat. R. Astr
     Soc. 243, 557-564 (1990). [BIBCODE 1990MNRAS.243..557B ]
Budding, E. and Zeilik, M., Ap. J. 319, 827-835 (1987). [BIBCODE 1987ApJ...319..827B ]
Heckert, P. and Zeilik, M, IBVS No. 3253 (1988).
Laubscher, B., Gregory., Bauer, T., Zeilik, M, and Burns, J., Pub. Astron Soc.
     Pacific, 100, 131-136 (1988). [BIBCODE 1988PASP..100..131L ]
Strassmeier K.G., Hall D.S., Zeilik, M. Nelson, E., Eker, Z., Fekel, F.C.,
     Astron.  Astrophys. Suppl. Ser., 72, 291-345 (1988). [BIBCODE 1988A&AS...72..291S ]
Zeilik, M., Cox, D., De Blasi, C., Ledlow, M., Rhodes, M., and Williams, T.,
     IBVS, No. 3200 (1988).