Abstract
We review arguments suggesting that millisecond variability detected in the X-ray flux of LMXBs has its origin in oscillation modes of the accretion disk. The twin high-frequency QPOs observed in microquasars seem to be related to the twin kHz QPOs in neutron-star systems, and both phenomena are likely the result of a non-linear resonance in the accretion disk that is possible only in strong-field gravity. A presumed 3:2 eigenfrequency ratio of the resonant modes corresponds to the 3:2 QPO frequency ratio clearly detected in black-hole systems, as well as to the more complex distribution of kHz QPO frequency pairs in neutron stars.
