Single and Composite Hot Subdwarf Stars in the Light of 2MASS Photometry.

Utilizing the Two Micron All Sky Survey (2MASS) All-Sky Data Release Catalog, we have retrieved useful near-IR J, H and KS magnitudes for more than 800 hot subdwarfs (sdO and sdB stars) drawn from the Catalogue of Spectroscopically Identified Hot Subdwarfs (Kilkenny, Heber, and Drilling, 1988, 1992). This sample size greatly exceeds previous studies of hot subdwarfs. We find that of the hot subdwarfs in Kilkenny et al., ∼40% in a magnitude-limited sample have colors that are consistent with the presence of an unresolved late-type (FGK) companion. Binary stars are over-represented in a magnitude-limited sample. In an approximately volume-limited sample the fraction of composite-color binaries is ∼25%. [ABSTRACT FROM AUTHOR]