1997PASP..109..461F

We have obtained 18 moderate-resolution (∼ 2.5Å) spectra of the Galactic X-ray Nova Ophiuchi 1977 in quiescence with the first of the W. M. Keck 10∼m telescopes. Cross-correlation with the spectra of late-type dwarfs (especially K7) yields excellent radial velocities for the companion star. The orbital period (P) is 0.5229 ± 0.0044 d, and the semiamplitude (K_c) is 447.6 ± 3.9 km.s^–1; these are consistent with the values previously derived by Remillard et al. (1996) from CTIO photometry (P = 0.5213 ± 0.0013 d) and spectroscopy (K_c = 420 ± 30 km.s^–1). Our derived mass function, f(M_x) = PK_c^3/2 π G, is 4.86 ± 0.13 M_☉ (cf. 4.0 ± 0.8, Remillard et al.), the third highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (∼ 3.2 M_☉)), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on an inclination i = 70° (from ellipsoidal modulations) and M_c = 0.3-0.6 M_☉ are M_x = 6.4 - 6.9 M_☉.