  We report optical spectroscopic observations of the Low Mass X-ray Binary GRO
J1655-40 during outbursts in August-September 1994 and in June 1996. The
spectra show emission and absorption lines from the accretion 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 first time among black-hole candidates that
an emission-line radial velocity curve is seen in phase with the expected
motion of the primary. The projected radial velocity semi-amplitude determined
for the primary [K(X) = 76.2 +/- 7.5 km/s], combined with estimates of the
projected radial velocity semi-amplitude of the companion star from previous
observations [K(C) = 228.2 +/- 3.0 km/s], yields a 95% confidence limit,
derived purely from kinematics, of M(X) > 5.1 M(sun) for the mass of the
compact object in GRO J1655-40, confirming its black hole nature.
