  We propose a simple and unambiguous way to deduce the parameters of black
holes which may reside in AGNs and some types of X-ray binaries. The black-hole
mass and angular momentum are determined in physical units. The method is
applicable to the sources with periodic components of variability, provided one
can assume the following: (i) Variability is due to a star or a stellar-mass
compact object orbiting the central black hole and passing periodically through
an equatorial accretion disk (variability time-scale is given by the orbital
period). (ii) The star orbits almost freely, deviation of its trajectory due to
passages through the disk being very weak (secular); the effect of the star on
the disk, on the other hand, is strong enough to yield observable photometric
and spectroscopic features. (iii) The gravitational field within the nucleus is
that of the (Kerr) black hole, the star and the disk contribute negligibly.
