1999A&AS..134....1S

A detailed analysis of the period changes of sixteen semi-detached binaries which contain only stars with radiative outer layers (ET-systems) has been performed and their respective O-C diagrams are brought and discussed. It was found that the course of the period variations is monotonic and in some systems the period is even constant. This detailed analysis of extensive sets of timings covering several decades brings a strong support to an earlier finding of Hall (1989) and develops it further. We show that all systems with the orbital periods shorter than nine days display constant period or its increase, with the exception of an uncertain case of V337 Aql. The course of the period variations in TT Aur appears more complex but the cyclic term can be plausibly explained by the third body. A search for general relations between the parameters of the systems and the period changes was undertaken. The mass ratio q appears to play a role in the period variations. Clear changes in systems with orbital periods shorter than 9 days were detected only for q>0.4 while constant periods are common in systems with q<0.4 in this period range. The sense of the secular changes in most systems is in accordance with the dominant conservative mass transfer in evolved binary and corresponds to the slow phase. The role of the evolutionary scenarios (case A versus B) and the influence of changes of the rotational angular momenta of the components are discussed.