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We have continued optical observations of the black hole candidate X-ray Nova Muscae 1991 (=GRS 1124-683, GU Mus) in quiescence over the last three years. These observations have refined the measurements of the orbital period ($P=0.4326051\pm 0.0000038$ days) and the sinusoidal absorption-line radial velocity curve (half-amplitude of $K_2=406\pm 7$ km sec$^{-1}$). The implied mass function is $f(M)=3.01\pm 0.15\,M_{\odot}$, which represents the minimum mass of the compact primary. A strong lower limit on the mass of the primary based on the lack of observed optical and X-ray eclipses and a previous measurement of the binary mass ratio is $M_1>4.1\,M_{\odot}$; this mass exceeds the maximum mass of a stable neutron star, reinforcing the argument that the Nova Muscae system contains a black hole. The phase-averaged spectrum (Doppler corrected to account for the orbital motion of the secondary) shows strong absorption lines consistent with a dwarf main sequence star with an MK type of K3V-K5V. The disk spectrum, obtained by subtracting the spectrum of a K5V template star from the averaged spectrum, shows strong Balmer emission lines, as well as emission lines of He I and Fe II, which are superimposed on a relatively flat continuum.
We have also monitored the optical outburst behavior of Nova Vela 1993 (=GRS 1009-45) and Nova Sco 1994 (=GRO J1655-40). Nova Vela has several ``mini-outbursts'' in its light curve similar to those seen in the light curve of the black hole candidate X-ray Nova Per 1992 (=GRO J0422+32). During one of the mini-outbursts, a large-amplitude optical modulation was clearly seen with a period of approximately 0.16 days. The light curve for X-ray Nova Sco 1994 is much less complete; however, there is marginal evidence for an eclipse. If the eclipse interpretation is correct, the orbital period is likely to be greater than about 1.3 days.
