Relation of fraction of binary stars with spectral class (mass) What is relation of fraction of binary stars with spectral class (mass)? For example, how many binary stars are among O,B,A,F,G,K,M stars separately?
 A: The second paragraph of this paper by Kouwenhoven et al. has a great summary:

Practically all O-type stars (Mason et al. 1998) and B/A-type stars (Shatsky & Tokovinin 2002; Kobulnicky & Fryer 2007; Kouwenhoven et al. 2007b) are found in binary or multiple systems. Abt & Levy (1976) report a multiplicity fraction of 55% among F3−G2 stars, and in their CORAVEL spectroscopic study of F7−G9 stars, Duquennoy & Mayor (1991) find a binary fraction of ∼60%. The binary fraction among M-type stars is 30−40% (Fischer & Marcy 1992; Leinert et al. 1997; Reid & Gizis 1997). For late M-type stars and brown dwarfs the binary fraction decreases to 10−30% (e.g., Gizis et al. 2003; Close et al. 2003; Bouy et al. 2003; Burgasser et al. 2003; Siegler et al. 2005; Ahmic et al. 2007; Maxted et al. 2008; Joergens 2008).

(Note those references can be found easily on the SAO/NASA ADS here.)
The rest of the paper describes some of the subtleties at work when astronomers combine modeling with observational data.
A: One should be careful about the selection effects here.  For example for an F star to have an M companion, we call it an F star in a binary system.  But when a sample of M stars is compiled, people looked for companions fainter than the main targets...
