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Published online by Cambridge University Press: 14 August 2015
Accumulated evidence on the existence of collapsed stars (black holes) in X-ray and non-X-ray binaries is considered. At present Cyg X-1 is the only strong case. The determinacy of solutions for mass ratios and other parameters of X-ray binaries is discussed, and some effects of tidal distortion and X-ray heating on measured radial velocities and ellipsoidal light curves are described.