  Complex period variations of five W UMa type binaries (AB And, OO Aql, DK
Cyg, V566 Oph, U Peg) were investigated by analyzing their O--C diagrams, and
several common features were found. Four of the five systems show secular
period variations at a constant rate on the order of |dP / dt 1/P | ~ 10E-7
1/yr. In the case of AB And, OO Aql, and U Peg a high-amplitude, nearly
one-century long quasi-sinusoidal pattern was also found. It might be explained
as light-time effect, or by some magnetic phenomena, although the mathematical,
and consequently the physical, parameters of these fits are very problematic,
as the obtained periods are very close to the length of the total data range.
The most interesting feature of the studied O--C diagrams is a low amplitude
(2-4 x 10E-3 d) modulation with a period around 18--20 yr in four of the five
cases. This phenomenon might be indirect evidence of some magnetic cycle in
late-type overcontact binaries as an analog to the observed activity cycles in
RS CVn systems.
