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

Years to decade-long cyclic orbital period changes have been observed inseveral classes of close binary systems including Algols, W Ursae Majoris andRS Canum Venaticorum systems, and the cataclysmic variables. The origin ofthese changes is unknown, but mass loss, apsidal motion, magnetic activity, andthe presence of a third body have all been proposed. In this paper we use newCCD observations and the century-long historical record of the times of primaryeclipse for WW Cygni to explore the cause of these period changes. WW Cygni isan Algol binary whose orbital period undergoes a 56 year cyclic variation withan amplitude of approximately 0.02 days. We consider and reject the hypothesesof mass transfer, mass loss, apsidal motion and the gravitational influence ofan unseen companion as the cause for these changes. A model proposed byApplegate, which invokes changes in the gravitational quadrupole moment of theconvective and rotating secondary star, is the most likely explanation of thisstar's orbital period changes. This finding is based on an examination of WWCygni's residual O-C curve and an analysis of the period changes seen in 66other Algols. Variations in the gravitational quadrupole moment are alsoconsidered to be the most likely explanation for the cyclic period changesobserved in several different types of binary systems.Comment: Accepted to AJ, 23 pages, 7 postscript figure