Cygnus X-1 is an X-ray binary that was one of the first X-ray sources discovered when it was detected in 1962. The visible object HDE226868 is a 9th magnitude blue supergiant star whose radial velocity curve shows an orbital period of 5.6 days. The fact that the object is a strong X-ray emitter and that the optical and X-ray emission varies on very short time scales (as short as one one-thousandth of a second) suggest that the companion might be a black hole. Analysis of the radial velocity variation of the primary under the assumption that it is a normal star suggests that the mass of the companion is about 6 solar masses.

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