Parameters of black holes in sources with periodic variability

We propose a way to deduce the parameters of accreting black holes. The met hod employs the properties of the spectral features observed in radiation f rom an accretion disk. It is applicable to sources which exhibit periodic m odulation of variability, provided: (i) the gravitational field is determin ed by the black hole and described by the Kerr metric; (ii) a thin accretio n disk of negligible mass lies in the equatorial plane of the hole; (iii) a secondary object (also with negligible mass) moves on a slightly inclined almost circular orbit around the black hole and passes periodically through the disk; (iv) the collisions result in observable photometric and spectro scopic features (temporal variability of the radiation flux and of spectral -line profiles produced in the disk), which show frequencies of the orbital motion and of latitudinal oscillations; (v) one can measure the width of t he spectral line from hot spots arising in the disk due to collisions with the orbiter, and/or detect predicted low-frequency oscillations induced in the disk. These frequencies and the line width provide enough information t o determine in physical units three parameters characterizing the source: t he mass and angular momentum of the central black hole, and radius of the o rbit of the secondary.