An active binary XY UMa revisited

New extensive multicolour photoelectric photometry, performed since 1994, is presented. 17 more- or-less complete light curves were obtained and analyzed. The Wilson-Devinney code applied to the BVRI light curves without the maculation eect together with published spectroscopic mass ratio and semi-major axis yielded new absolute parameters of the eclipsing pair: m1 =1 :10 M, m2 =0 :66 M, R1 =1 :16 R, R2 =0 :63 R, a =3 :107 R and the orbital inclination i =8 0:9. The observed orbital period changes are conclusively explained by the mutual action of the third body in the system (P3 = 30 years) and the maculation eects. Simultaneous analysis of the period changes and the visual brightness excludes the possibility of their explanation by Applegate's mechanism. The dierences in the maxima heights caused by the maculation exhibit variations with the period of 70910 days. The distance to the system d =8 65 pc determined from the absolute dimensions and luminosities of the components is larger than the Hipparcos astrometric value d =6 6 6p c.