  Millisecond variability detected in low-mass X-ray binaries (LMXBs) probably
reflects motions of accreting fluid in the strong-field gravity of neutron
stars or neutron stars. Parametric resonance between two oscillators with
unequal eigenfrequencies provides a natural explanation for the 2:3 frequency
ratio observed in some black-hole systems, and probably also in the brightest
neutron stars. The two oscillators likely correspond to the meridional and
radial epicyclic motions.
