COMMISSION 27 OF THE I.A.U.
               INFORMATION BULLETIN ON VARIABLE STARS
                            Number 2946

                                                    Konkoly Observatory
                                                    Budapest
                                                    16 October 1986

                                                    HU ISSN 0374 - 0676


          THE X-RAY LIGHT CURVE OF XY URSAE MAJORIS

XY UMa is a short period (P = 0.48d) active late-type (G2-G5 +
K5) detached eclipsing binary system, in which the hotter star
appears to be the active component (Geyer 1980). We expected
such a system to be a source of coronal X-ray emission, and this
expectation was supported by the strength of the chromospheric
ultraviolet emission lines (Budding, Kadouri and Gimenez 1982).
Observations with the EXOSAT X-ray observatory were made
continuously through 1.25 orbital cycles during 1986 March 16/17.

                        [FIGURE 1]

We present here the soft X-ray light curve, obtained with the LE
CMA instrument through the thick Lexan filter,  which has a
response over the energy band 0.04-2 keV (6-300 A). The phase
is calculated from the ephemeris of Geyer (1977)

        HJD = 2435216.5011 + 0.4789944d E

and the signal is in units of photon count: per time bin of 1035s
(40 bins = 1 orbital period).
   It is seen that XY UMa shows considerable and complex variability
in its X-ray emission over the orbital cycle, with evidence of a
primary eclipse but absence of a secondary eclipse as well as
flaring activity. A full analysis of the X-ray light curve is in
progress and will be published elsewhere.


   D. K. BEDFORD                      O. VILHU
  Dept of Space Research     Observatory and Astrophysics
University of Birmingham            Laboratory.
       PO Box 363              University of Helsinki
  Birmingham B15 2TT, UK         SF00130 Helsinki 13
              UK                      Finland

                  J. J. FUENSALIDA
          Instituto do Astrofisica de Canarias
                  38071 La Laguna
                     Tenerife
                  Canary Islands


References:

Budding, E., Kadouri. T.H., and Gimenez A. 1982. Astrophys. Space. Sci. 88, 453 [BIBCODE 1982Ap&SS..88..453B ]
Geyer, E.H. 1977, Astrophys. Space Sci. 48, 137. [BIBCODE 1977Ap&SS..48..137G ]
Geyer, E.H. 1980, "Close binary stars: observations and interpretation", ed.
     Plavec, M.J., Popper, D.M. and Ulrich, R. K. (Dordrecht, Reidel). [BIBCODE 1980IAUS...88..423G ]