 As a main-sequence star goes through an activity cycle, the outer layers of the star are subject to a magnetic torque changing the distribution of angular momentum, resulting in a change in the star's oblateness.The orbit of the stars in the binary pair is gravitationally coupled to their shape changes, so that the period shows modulations (typically on the order of ∆P/P ∼ 10−5) on the same time scale as the activity cycles (typically on the order of decades).Another phenomenon observed in some Algol binaries has been monotonic period increases.