  The advent of the large effective apertures of the Keck telescopes has
resulted in the determination with unprecedented accuracy of the mass functions
and mass ratios of faint (R ~ 21 mag) X-ray transients (GS 2000+25, GRO
J0422+32, Nova Oph 1977, Nova Vel 1993), as well as constraining the
main-sequence companion star parameters and producing images of the accretion
disks around the black holes.
