Constraints on neutron star matter from kilohertz QPOs
  One of the most dramatic discoveries made so far with the Rossi X-Ray Timing Explorer is that many accreting neutron stars with weak magnetic fields generate strong, remarkably coherent, high-frequency X-ray brightness oscillations. The 325-1200 Hz quasi-periodic oscillations (QPOs) observed in the accretion-powered emission are almost certainly produced by gas orbiting very close to the stellar surface and have frequencies related to the orbital frequencies of the gas. The 360-600 Hz brightness oscillations seen during thermonuclear X-ray bursts are produced by one or two hotter regions on the stellar surface and have frequencies equal to the stellar spin frequency or its first overtone. Measurements of these oscillations are providing tight upper bounds on the masses and radii of neutron stars, and important new constraints on the equation of state of neutron star matter. 
F. K. Lamb, M. C. Miller and D. Psaltis