A BLACK-HOLE IN THE X-RAY NOVA GS-2000+25

We have obtained 13 moderate-resolution (similar to 2.5 Angstrom) spectra of the Galactic X-ray nova GS 2000+25 in quiescence with the W. M. Keck 10 m telescope. Cross-correlation with the spectra of late-type dwarfs (especially K2-K7) yields excellent radial velocities for the secondary star. The orbital period is consistent with that previously obtained from photometry (similar to 8.3 hr). A semiamplitude of 518.4 +/- 3.5 km s(-1) is found, similar to 25% smaller than the outer disk velocity implied by the double-peaked H alpha emission line (Delta v/2 approximate to 700 km s(-1)). The derived mass function is 4.97 +/- 0.10 Mo, the second highest known for any low-mass X-ray binary. Being considerably above the conventional limiting mass for a neutron star (similar to 3.2 M.), it strongly suggests that the compact primary is a black hole. Plausible mass estimates based on M(2) = 0.4-0.7 M. and i = 67 degrees 5-80 degrees are M(1) = 5.9-7.5 M.. Under the assumption that the radial velocity curve of the centroid of the Ha emission line represents the true motion of the black hole, we formally find q = M(2)/M(1) = 0.050 +/- 0.026.