Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, U.S.A.

There is growing evidence that a significant number of the rapidly rotating Be stars were spun up through mass transfer in massive close binary stars. We observe Be stars in several kinds of binaries predicted by theory: (1) Be + a cool, Roche-filling companion (so-called ``hot Algols''), (2) Be + He star (the stripped down core of the mass donor), and (3) Be + neutron star (Be X-ray binaries). There are no known examples of Be + white dwarf binaries (although EUVE observations of the system  Sco may have revealed the first rapidly rotating B star + white dwarf binary). Here I review the observational data on Be stars in binaries for comparison with model predictions. Because the detection of hot, faint (He star and white dwarf) companions is so difficult, we still cannot estimate accurately the fraction of such systems among the Be star population. Thus, the actual percentage of Be stars formed by binary mass transfer processes remains unknown.

To appear in "The influence of binaries on stellar population studies" ed. D. Vanbeveren (Dordrecht: Kluwer)
Preprints from gies@chara.gsu.edu
or on the web at http://www.chara.gsu.edu/~gies/be.ps