  The activity of at least one repeating Fast Radio Burst (FRB) source is
periodically modulated. If this modulation is the result of precession of the
rotation axis and throat of an accretion disc around a black hole, driven by a
companion that is also the source of accreted mass, then it may be possible to
constrain the mass of the black hole. The dynamics is analogous to that of
superorbital periods in ordinary mass-transfer binaries in which the accreting
object may be a stellar-mass black hole, a neutron star or a white dwarf, but
in the FRB source it may be an intermediate mass black hole. In a semi-detached
(mass-transferring) binary the orbital period is related to the mean density of
the mass-losing star. Assuming a value for its density and identifying the
observed modulation period as a disc precession period would determine the mass
ratio and the mass of the black hole. This model and magnetar-SNR models make
distinguishable predictions of the evolution of the FRB rotation measure that
may soon be tested in FRB 121102.
