Spectroscopic studies of O type stars. IX. Binary frequency.

Using 243 new coude spectrograms, a sample of 67 O type stars was studied to determine the frequency of binaries. Binaries are detected in 36 percent of the sample, of which 85 percent have mass ratios less than 2.5. The binary frequency is lower than in earlier studies because a number of stars, previously identified as variable, show only random photospheric variations up to 30 km/s. It appears that mass ratios greater than 3 may not be present in unevolved O type systems. The relationship of the highly evolved X-ray binary systems to the unevolved O type is discussed. From statistical arguments, a few of the 67 O stars in the sample could contain neutron star companions and several candidates are listed.