  The frequencies of the highest-frequency kilohertz QPOs recently discovered
in some sixteen neutron stars in low-mass X-ray binary systems are most likely
orbital frequencies. If so, these QPOs provide tight upper bounds on the masses
and radii of these neutron stars and interesting new constraints on the
equation of state of neutron star matter. If the frequency of a kilohertz QPO
can be established as the orbital frequency of the innermost stable circular
orbit, this would confirm one of the key predictions of general relativity in
the strong-field regime. If the spin frequency of the neutron star can also be
determined, the frequency of the QPO would fix the mass of the neutron star for
each assumed equation of state. Here we describe how bounds on the stellar mass
and radius can be derived and how these bounds are affected by the stellar
spin. We also discuss detection of the innermost stable circular orbit.
