  In all four microquasars which show double peak kHz QPOs, the ratio of the
two frequencies is 3:2. This strongly supports the suggestion that twin peak
kHz QPOs are due to a resonance between some modes of accretion disk
oscillations. Here, we stress that fits to observations of the hypothetical
resonances between vertical and radial epicyclic frequencies (particularly of
the parametric resonance) give an accurate estimate of the spin for the three
microquasars with known mass. Measurement of double peak QPOs frequencies in
the Galaxy centre seems also to be consistent with the 3:2 ratio established by
previous observations in microquasars, however the Sgr A* data are rather
difficult for the same exact analysis. If confirmed, the 3:2 ratio of double
peak QPOs in Sgr A* would be of a fundamental importance for the black hole
accretion theory and the precise measurement could help to solve the question
of QPOs nature.
