MEASURING THE MOTION OF THE BLACK-HOLE IN GRO J1655-40

We report optical spectroscopic observations of the low-mass X-ray bin ary GRO J1655-40 during outbursts in 1994 August-September and in 1996 June. The spectra show emission and absorption lines from the accreti on disk surrounding the X-ray source, which have enabled us to measure the motion of the black hole candidate in this system. It is the firs t time among black hole candidates that an emission-line radial veloci ty curve is seen in phase with the expected motion of the primary. The projected radial velocity semiamplitude determined for the primary (K -X = 76.2 +/- 7.5 km s(-1)), combined with estimates of the projected radial velocity semiamplitude of the companion star from previous obse rvations (K-C = 228.2 +/- 3.0 km s(-2)), yields a 95% confidence limit , derived purely from kinematics, of M-X > 5.1 M. for the mass of the compact object in GRO J1655-40, confirming its black hole nature.