  Stars with an initial mass more than ~25 Msun are thought to ultimately
become black holes. Then stellar-mass black holes should be ubiquitous but
fewer than 20 have been found in our Galaxy to date, all of which have been
found through their X-ray emission. In most cases these are soft X-ray
transients --- low-mass X-ray binaries whose optical counterparts are late type
stars filling their Roche lobes, leading to accretion onto black holes. In one
case, the stellar-mass black hole is in a high-mass X-ray binary whose optical
counterpart is an early type star. Its strong stellar winds are accreted by the
black hole, producing X-ray emission. It follows that X-ray-quiet stellar-mass
black holes exist in wide binary systems. The discovery of black holes in the
optical through their gravitational interactions would be a major scientific
breakthrough. Recent space-based photometry has made it possible to measure
phase or frequency modulation of pulsating stars to extremely high precision.
Such modulation is caused by orbital motion, and its analysis offers the lower
limit for the mass of the companion to the pulsating star. If the companions
are non-luminous and if the masses of the companions exceed the mass limit for
neutron stars (~3 Msun), the companions should be black holes. We review the
methodology, and analyses of some encouraging cases are demonstrated.
