COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3374

                                        Konkoly Observatory
                                        Budapest
                                        19 September 1989

                                        HU ISSN 0374 - 0676


     PHOTOELECTRIC LIGHT CURVES AND MINIMUM TIMES OF XY UMA


XY UMa (BD+55d1317) is a binary system of RS CVn type with a period of
0.479.d The star has been known to exhibit large and long-term changes of its
light curve and brightness and this was interpreted by Geyer (1980) and Jassur
(1986).

In order to monitor its light curve variations, we observed the system
photoelectrically with the 60 cm reflector and a single channel photometer at
the Xinglong Station of Beijing Observatory. The observations were made in two
sets, during three nights from 7 to 9 March and again during four nights from
30 March to 2 April, 1989. The stars BD+54d1278 and BD+54d1275 were used as
the comparison and check star, respectively.

The first and second light curves sets and (B-V) color index curves are shown
in Fig. 1 and Fig. 2, respectively. Five primary minimum times were obtained.
They are given in Table 1.

Table 1.   Times of minima of XY UMa

           J.D.Hel.   Filter   m.e.
        2447593.2492  B        0.0001
            593.2489  V        0.0002
        2447594.2069  B        0.0002
            594.2067  V        0.0001
        2447616.2404  B        0.0002
            616.2398  V        0.0005
        2447617.1979  B        0.0001
            617.1978  V        0.0003
        2447618.1553  B        0.0002
            618.1556  V        0.0003

Comparing Fig. 1 with Fig. 2, we find the light curve in Fig. 1 to be symmetrical,
but that in Fig. 2 is asymmetrical. There is a brightening of 0.04 mag around
phase 0.25 within a month, which may be caused by asymmetric brightness
distributions on the disk of the primary component.


                            [FIGURE 1]
                            [FIGURE 2]


Combining Table 1 with the other 33 minima given by Hall and Kreiner (1980), we
have derived the following linear and quadratic ephemerides:

Min.I(Hel.) = 2439913.5245+0.47899468d E        (1)
                       +-7        +-5

Min.I(Hel.) = 2439913.5216+0.47899478d E + 1.4 * 10^-11 E^2 (2)
                      +-10        +-5      +-3

It is shown that the new average period of XY UMa is 0.47899468d in ephemeris
(1).  However, the (O-C) residuals of the ephemeris (1) show a long-term
change. Fitting with the quadratic ephemeris (2) we find that the period of XY
UMa seems to be increasing at a rate of Delta P/P ~1.8 * 10^-3 sec/yr from
1931 up to the present.

The photometric analysis will be presented in another paper.


LI QISHENG
ZHANG XIAOYU
ZHANG RONGXIAN
Beijing Astronomical Observatory
Academia Sinica
Beijing, China

References:

Geyer, Edward H., 1980, in Close Binary Stars: Observations and Interpretation,
   IAU symp. No. 88, 423. [BIBCODE 1980IAUS...88..423G ]
Jassur, D.M.Z., 1986, Astrophys. Space Sci., 128, 369. [BIBCODE 1986Ap&SS.128..369J ]
Hall, D.S. and Kreiner, J.M., 1980, Acta Astron., 30, 387. [BIBCODE 1980AcA....30..387H ]