At orbital periods greater than about 30 days, the amount of tidal synchronization decreases. This transition period is in good agreement with the predicted and observed circularization period for Milky Way field binaries. At 10 days, there is a transition from predominantly circular, synchronized EBs to predominantly eccentric, pseudosynchronized EBs. However, a population of short period EBs exists with rotation periods typically 13% slower than synchronous, which we attribute to the differential rotation of high latitude starspots. We report rotation periods for 816 EBs with starspot modulations, and find that 79% of EBs with orbital periods less than ten days are synchronized. We visually inspected the light curves of 2278 eclipsing binaries (EBs) from the Kepler Eclipsing Binary Catalog to identify those with starspot modulations, as well as other types of out-of-eclipse variability. Few observational constraints exist for the tidal synchronization rate of late-type stars, despite its fundamental role in binary evolution.
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