Abstract

Binary companions likely affect stellar rotational evolution via tides (for close binaries) and disruption of the protoplanetary disk (for binary semimajor axes out to at least 5 to 80 AU). However, models of angular momentum evolution assume that all stars are single, and tidal models for solar-type stars cannot explain the handful of existing observations. Binary surveys in open clusters are woefully incomplete, particularly for K and M stars and stars with measured rotation periods. I will present preliminary results showing that rapidly rotating M dwarfs in the Praesepe cluster tend to have companions within 80 AU, in agreement with other recent surveys. I will also discuss ongoing multiplicity surveys of open clusters using a variety of detection methods, including Keck adaptive optics, spectroscopy, and/or Gaia RUWE.