From U, B, V photometry of 104 stars in a field of radius 30' centered on the X-ray binary star Cyg X-1 (HDE 226868), we have studied the color excess E(B - V) as a function of distance. Spectral types were observed de novo for 42 of these stars. We conclude that HDE 226868 cannot be nearer than 1 kpc, and is probably at a distance of 2.5 kpc or more. The primary component is therefore a luminous OB star of mass 30 , and the X-ray component has a minimum mass Subject headings:binaries - black holes - X-ray sources
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Bregman et al. (1973) studied this question.