The mass ratio of Nova Muscae 1991

We report intermediate resolution H alpha spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85+/-6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106+/-13 km s(-1) with respect to template field stars, leading to a system mass ratio of q = M(1)/M(2) = 7.8(-2.0)(+3.4). Doppler imaging of the H alpha line shows strong emission coming from the secondary star (EW = 3.1+/-0.6 Angstrom) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods, The surface brightness distribution of the accretion disk in H alpha follows a relation I proportional to R(-1.1), less steep than typically observed in cataclysmic variables, Updated system parameters are also presented.