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We present results of a high angular resolution survey of massive OB stars inthe Cygnus OB2 association that we conducted with the Fine Guidance Sensor 1R(FGS1r) on the Hubble Space Telescope. FGS1r is able to resolve binary systemswith a magnitude difference delta-V < 4 down to separations as small as 0.01arcsec. The sample includes 58 of the brighter members of Cyg OB2, one of theclosest examples of an environment containing a large number of very young andmassive stars. We resolved binary companions for 12 targets and confirmed thetriple nature of one other target, and we offer evidence of marginally resolvedcompanions for two additional stars. We confirm the binary nature of 11 ofthese systems from complementary adaptive optics imaging observations. Theoverall binary frequency in our study is 22% to 26% corresponding to orbitalperiods ranging from 20 - 20,000 years. When combined with the knownshort-period spectroscopic binaries, the results supports the hypothesis thatthe binary fraction among massive stars is > 60%. One of the new discoveries isa companion to the hypergiant star MT 304 = Cyg OB2-12, and future measurementsof orbital motion should provide mass estimates for this very luminous star.

A High Angular Resolution Survey of Massive Stars in Cygnus OB2: Results from the Hubble Space Telescope Fine Guidance Sensors