  Many Galactic black hole and neutron star sources in low X-ray mass binaries
show QPOs (quasi periodic oscillations) in their observed X-ray fluxes, i.e.
peaks in the Fourier variability power spectra. Pairs of twin peaks are
observed, and in black-hole systems their frequencies (upp), (down) are in
rational ratios. For example, in all four microquasars with twin peaks
observed, (upp):(down) = 3:2. The rational ratios have been postulated in a
model that explained twin peak QPOs as a non-linear resonance between modes of
accretion disk oscillations. For those microquasars where the mass of the X-ray
source is known, we determine the black-hole spin, following from the observed
QPO frequencies within various models of resonance.
