  The optical counterpart of the black-hole soft X-ray transient Nova Muscae
1991 has brightened by $\Delta{V}\approx0.8$ mag since its return to quiescence
23 years ago. We present the first clear evidence that the brightening of soft
X-ray transients in quiescence occurs at a nearly linear rate. This discovery,
and our precise determination of the disk component of emission obtained using
our $simultaneous$ photometric and spectroscopic data, have allowed us to
identify and accurately model archival ellipsoidal light curves of the highest
quality. The simultaneity, and the strong constraint it provides on the
component of disk emission, is a key element of our work. Based on our analysis
of the light curves, and our earlier measurements of the mass function and mass
ratio, we have obtained for Nova Muscae 1991 the first accurate estimates of
its systemic inclination $i=43.2^{+2.1}_{-2.7}$ deg, and black hole mass
$M=11.0^{+2.1}_{-1.4}\ M_\odot$. Based on our determination of the radius of
the secondary, we estimate the distance to be $D=4.95^{+0.69}_{-0.65}$ kpc. We
discuss the implications of our work for future dynamical studies of black-hole
soft X-ray transients.
