  In all the microquasars with two hHz QPOs, the ratio of the frequencies is
3:2, supporting our suggestion that a non-linear resonance between two modes of
oscillation in the accretion disk plays a role in exciting the observed
modulations of the X-ray flux. We discuss the evidence in favor of this
interpretation, and we relate the black hole spin to the frequencies expected
for various types of resonances that may occur in nearly Keplerian disks in
strong gravity. For those microquasars where the mass of the central X-ray
source is known, the black hole spin can be deduced from a comparison of the
observed and expected frequencies.
