The X-rays emitted by accreting black holes and neutron stars are modulated quasi-periodically at very high frequencies. These kHz Quasi Periodic Oscillations (QPOs) often occur in pairs of frequencies that in black hole sources are in a 3:2 ratio. The frequencies likely represent two resonant oscillations of the accretion disk. We have evidence that the neutron star data point to the same ratio, and arguments suggesting that the 3:2 frequency ratio may be a signature of strong gravity. The modulation itself of the X-ray light curve may be caused by another effect of general relativity: light-bending close to the black hole. In some neutron star sources, the difference of the two kHz frequencies is very nearly equal to one-half of the stellar spin frequency, which is a clear signature of a non-linear resonance excited by the rotating neutron star.

PACS numbers: 04.70.Bw, 52.35.Mw, 95.30.Lz, 97.60.Jd