Taking the Matese and Whitmire mechanism as a basis, Applegate argued that changes in the radius of gyration of one star could be related to magnetic activity cycles. Supportive evidence for his hypothesis came from the observation that a large fraction of the late-type secondary stars of Algol binaries appear to be rapidly rotating convective stars, implying that they should be chromospherically active. Indeed, orbital period modulations are seen only in Algol-type binaries containing a late-type convective star.