  In the inner regions of accretion disks around compact objects, the orbital
frequency of the gas deviates from the local Keplerian value. For
long-wavelength modes in this region, the radial epicyclic frequency kappa is
higher than the azimuthal frequency Omega. This has significant implications
for models of the twin kHz QPOs observed in many neutron-star sources that
traditionally identify the frequencies of the two kHz QPOs with dynamical
frequencies in the accretion disk. The recognition that the highest frequency
in the transition or boundary region of the disk is actually the epicyclic
frequency also modifies significantly the constraints imposed by the
observation of high-frequency QPOs on the mass and radius of the compact
objects.
