2000A&AS..146..377Q

Orbital period variations of six semi-detached binaries, UW Cyg, RX Hya, AK Ser, AC Tau, UW Vir and VV Vul, are presented based on the analysis of their O-C observations. For AK Ser and VV Vul, their orbital periods may show secular increase at rates of dP/dt=+7.94x10^–7 and +3.43x10^–6 days/year. For UW Cyg, its orbital period is revised, and the O-C curve shows a cyclic variation with a period of 49.2years and an amplitude of 0.0383d. For the other three systems, RX Hya, AC Tau and UW Vir, the changes in their orbital periods appear more complex but their O-C curves can be represented by periodic variations (with periods of 54.3, 29.8 and 45.9years) superimposed on upward parabolic segments. The components of upward curving parabolic variations in RX Hya, AC Tau and UW Vir indicate secular period increase with rates of dP/dt=+1.04x10^–6, +7.11x10^–7 and +1.73x10^–6 days/year respectively. The secular period increase in the five systems, RX Hya, AK Ser, AC Tau, UW Vir and VV Vul, can be explained by mass transfer from the less to the massive components, which is consistent with their semi-detached configurations. The periodic changes of the orbital periods in UW Cyg, RX Hya, AC Tau, UW Vir may be caused by the light-time effects due to the existence of the third bodies.