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We present an examination of high resolution, ultraviolet spectroscopy fromHubble Space Telescope of the photospheric spectrum of the O-supergiant in themassive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-basedoptical spectra to determine the effective temperature and gravity of the O9.7Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), lineblanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. Theoptical spectrum is best fit with models that have enriched He and Nabundances. We fit the model spectral energy distribution for this temperatureand gravity to the UV, optical, and IR fluxes to determine the angular size ofand extinction towards the binary. The angular size then yields relations forthe stellar radius and luminosity as a function of distance. By assuming thatthe supergiant rotates synchronously with the orbit, we can use the radius -distance relation to find mass estimates for both the supergiant and black holeas a function of the distance and the ratio of stellar to Roche radius. Fits ofthe orbital light curve yield an additional constraint that limits thesolutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sunfor the supergiant and 11^{+5}_{-3} M_sun for the black hole.

The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1