OPTICAL OBSERVATIONS OF GRO J1655-40 IN QUIESCENCE .1. A PRECISE MASSFOR THE BLACK-HOLE PRIMARY

e report photometric and spectroscopic observations of the black hole binary GRO J1655-40 in complete quiescence. In contrast to the 1995 ph otometry, the light curves from 1996 are almost completely dominated b y ellipsoidal modulations from the secondary star. Model fits to the l ight curves, which take into account the temperature profile of the ac cretion disk and eclipse effects, yield an inclination of i=69 degrees .50+/-0 degrees.08 and a mass ratio of Q=M(1)/M=2.99+/-0.08. The preci sion of our determinations of i and Q allow us to determine the black hole mass to an accuracy of approximate to 4% (M(1)=7.02+/-0.22 M.). T he secondary star's mass is M(2)=2.34+/-0.12 M.. The position of the s econdary on the Hertzsprung-Russell diagram is consistent with that of a approximate to 2.3 M, star that has evolved off the main sequence a nd is halfway to the start of the giant branch. Using the new spectra, we present an improved value of the spectroscopic period (P=2(d).6215 7+/-0(d).00015), radial velocity semiamplitude (K=228.2+/-2.2 km s(-1) ), and mass function [f(M)=3.24+/-0.09 M.]. Based on the new spectra o f the source and spectra of several MK spectral type standards, we cla ssify the secondary star as F3 IV-F6 IV. Evolutionary models suggest a n average mass transfer rate for such a system of M(2)=3.4x10(-9) M. y r(-1)=2.16x10(17) g s(-1), which is much larger than the average mass transfer rates-implied in the other six transient black hole systems b ut is still barely below the critical mass transfer rate required for stability.