SPECTRAL VARIATIONS AND A CLASSICAL CURVE-OF-GROWTH ANALYSIS OF HDE 226868 (CYG X-1)

We present an analysis of high resolution (R similar to 17000) echelle spectra in the wavelength range 3200 to 6800 A obtained at the Observ atorio Astronomico Nacional at San Pedro Martir(2), of HDE 226868, the optical component of the Cyg X-1 binary system. Empirical radial velo city excitation (RV vs. E) relations are obtained from the spectra at 7 orbital phases and the slope (A) of these relations is found to vary , having two extrema: -0.30 km s(-1) eV(-1) near phase 0.5 (collapsed object ''in front'') and -0.22 km s(-1) eV(-1) near phase 0.0. At othe r phases the value is A = -0.13 km s(-1) eV(-1) This variability in th e slope of the RV vs. E relation is in very good agreement with the th eoretically predicted anisotropic mass-loss rates resulting from the m odified gravitational potential of the system (Friend and Castor 1982) for the case in which the primary star fills its Roche lobe. Variatio ns at the 25% level in the shapes and strengths of H beta and He I pho tospheric absorption lines are detected. The variability of the absorp tion line profiles appears to correlate with the H alpha and the He II 4686 A emission line variations, suggesting that the absorption lines are contaminated by variable emission components. From a classical cu rve-of-growth analysis HDE 226868 is found to have T-exc = 32100 +/- 1 600 degrees K, and to be overbundant in He and underabundant in C with respect to the B0Ia supergiant epsilon Ori.