Indirect evidence for short period magnetic cycles in W UMa stars

Period analysis of five overcontact systems

¹ Baja Astronomical Observatory of Bács-Kiskun County, 6500 Baja, Szegedi út, Kt. 766, Hungary e-mail: borko@alcyone.bajaobs.hu
² Dept. of Astronomy, National Research Institute of Astronomy and Geophysics (NRIAG), Helwan, Cairo, Egypt
³ Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, Pf. 67, Hungary
⁴ Department of Optics & Quantum Electronics & Astronomical Observatory, University of Szeged, 6701 Szeged, Pf. 406, Hungary
⁵ Guest Observer at Piszkéstető Station, Konkoly Observatory, Hungary

Received: 2 February 2005
Accepted: 24 May 2005

Abstract

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 yr^-1. 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 (~ 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.