Jerome A. Orosz coauthored a dynamical model of Cygnus X-1 that combines optical light curves, companion-star radial velocities, rotational broadening and a distance constraint to infer the black hole's mass.[ref]
The Mass of the Black Hole in Cygnus X-1 by Jerome A. Orosz and others. Abstract: photometric/spectroscopic constraints and binary model
The measurements constrain different parts of the binary geometry. Radial motion alone leaves an inclination dependence; stellar size and light-curve information help determine the scale and orientation. The quoted paper is a specific historical analysis, so its parameter estimates should not be mistaken for an automatically current calibration.
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