Abstract

New photoelectric UBV observations of the eclipsing contact binary systems U Peg, YY CrB, OU Ser and EQ Tau obtained from August 2000 to October 2001 are presented and analyzed. The (O-C) diagram of U Peg indicates a decrease of the orbital period combined with a quasi-periodic variation. In the case of EQ Tau the long-term period changes can be interpreted by the presence of a third body on a 50 year orbit. The present ephemerides for the studied systems were determined. The primary minimum of OU Ser, according to the Hipparcos photometry, is found to be the shallower one and hence the system belongs to the W subgroup. The photometric elements determined from our light curves combined with published spectroscopic elements yielded the absolute parameters of the systems.