  Years to decade-long cyclic orbital period changes have been observed in
several classes of close binary systems including Algols, 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 Cygni is
an Algol binary whose orbital period undergoes a 56 year cyclic variation with
an amplitude of approximately 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
Cygni'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.
