Our study shows that the orbital period variations of
XY UMa can be explained in principle by the appealing combination
of the Applegate gravitational coupling mechanism and
the mass-loss assumption. Existence of MgII emission lines
which is believed to be originated from the circumbinary
environment (Gurzadyan & Cholakyan 1995) supports this mass-loss
assumption. In order to verify the Applegate mechanism,
however, the magnetic activity of the primary component
of the system XY UMa should be monitored together
with the 
variation
and luminosity variation of the system with the same time scale.
The asymmetric light curves of XY UMa are
obtained in this work. Using the Wilson-Devinney program
(Wilson 1992), we determined a circular dark spot
which is supposed to take place on the
G2-G5V component of the system causing an asymmetry
in the light curves of the system. The photometric analysis
shows that XY UMa is a detached binary
where the primary component is an evolved object
and occupies 92% of its Roche lobe. These results agree
with the conclusion of Geyer (1977). Further, the
value of the semi-major axis of the apparent orbit
of the system (equals to 3.041 
; Geyer 1977)
was used to calculate the absolute parameters of the
components and the results are given in Table 9 (click here).
| Parameters | Primary | Secondary |
| component | component | |
Mass ( ) | 0.90 | 0.75 |
Mean Radius ( ) | 1.08 | 0.82 |
 |  |  |
| log g (cgs) | 4.33 | 4.48 |
|
|
Acknowledgements
The authors express their sincere thanks to Ömer Lütfi Degirmenci, Ahmet Devlen, Cengiz Sezer, Ömür Gülmen, Zeki Eker, Varol Keskin and Can Akan for many helpful discussions and valuable suggestions. This work was supported by The Research Foundation of Ege University under 1994/Fen/017.