The Origin of Cyclic Period Changes in Close Binaries: The Case of the Algol Binary WW Cygni

Year- to decade-long cyclic orbital period changes have been observed in several classes of close binary systems, including Algol, W Ursae Majoris, and RS Canum Venaticorum systems and the cataclysmic variables. The origin of these changes is unknown, but mass loss, apsidal motion, magnetic activity, and the presence of a third body have all been proposed. In this paper, we use new CCD observations and the century-long historical record of the times of primary eclipse for WW Cygni to explore the cause of these period changes. WW Cyg is an Algol binary whose orbital period undergoes a 56 yr cyclic variation with an amplitude of ~0.02 days. We consider and reject the hypotheses of mass transfer, mass loss, apsidal motion, and the gravitational influence of an unseen companion as the cause for these changes. A model proposed by Applegate, which invokes changes in the gravitational quadrupole moment of the convective and rotating secondary star, is the most likely explanation of this star's orbital period changes. This finding is based on an examination of WW Cyg's residual O-C curve and an analysis of the period changes seen in 66 other Algols. Variations in the gravitational quadrupole moment are also considered to be the most likely explanation for the cyclic period changes observed in several different types of binary systems.