Editing Binary star (section)

===Variations in period===
{{main|Applegate mechanism}}

The Applegate mechanism explains long term orbital period variations seen in certain eclipsing binaries. 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<sup>−5</sup>) on the same time scale as the activity cycles (typically on the order of decades).<ref name="Applegate 1992">{{cite journal |last1=Applegate |first1=James H. |title=A mechanism for orbital period modulation in close binaries|journal=Astrophysical Journal, Part 1 |date=1992 |volume=385 |pages=621–629|bibcode=1992ApJ...385..621A|doi = 10.1086/170967 |doi-access=free }}</ref>

Another phenomenon observed in some Algol binaries has been monotonic period increases. This is quite distinct from the far more common observations of alternating period increases and decreases explained by the Applegate mechanism. Monotonic period increases have been attributed to mass transfer, usually (but not always) from the less massive to the more massive star<ref name="Hall_1989">{{cite journal |last1=Hall |first1=Douglas S. |title=The relation between RS CVn and Algol |journal=Space Science Reviews |date=1989 |volume=50 |issue=1–2 |pages=219–233 |bibcode=1989SSRv...50..219H|doi = 10.1007/BF00215932 |s2cid=125947929 }}</ref>