  Kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries
(LMXBs) may represent the orbital frequencies of the innermost Keplerian orbits
around accreting neutron stars. Attempts have recently been made to derive
constraints on the mass and the equation of state of the neutron star, by
identifying the highest observed QPO frequency with the Keplerian frequency at
the marginally stable orbit given by the general theory of relativity. These
estimates have either neglected the effect of neutron star rotation or used an
approximate treatment of rotation in general relativity. We rederive these
constraints using a fully general relativistic formalism including the effect
of rapid rotation. We also present constraints corresponding to the case where
the innermost stable orbit touches the stellar surface.

Astrophysics