  Previous work on the sonic-point beat-frequency (SPBF) model of the kilohertz
quasi-periodic oscillations (QPOs) observed in the X-ray flux from neutron
stars in low-mass binary systems has shown that it naturally explains many
properties of these QPOs. These include the existence of just two principal
QPOs in a given source, the commensurability of the frequency separation \Dnu\
of the two kilohertz QPOs and the spin frequency \nus inferred from burst
oscillations, and the high frequencies, coherence, and amplitudes of these
QPOs. Here we show that the SPBF model predicts that \Dnu is less than but
close to \nus, consistent with the observed differences between \Dnu and \nus.
It also explains naturally the decrease in \Dnu with increasing QPO frequency
seen in some sources and the plateau in the QPO frequency--X-ray flux observed
in 4U 1820-30. The model fits well the QPO frequency behavior observed in Sco
X-1, 4U 1608-52, 4U 1728-34, and 4U 1820-30 (chisqdof = 0.4-2.1), giving masses
ranging from 1.59 to 2.0 Msun and spin rates ranging from 279 to 364 Hz. In the
SPBF model, the kilohertz QPOs are effects of strong-field gravity. Thus, if
the model is validated, the kilohertz QPOs can be used not only to determine
the properties of neutron stars but also to explore quantitatively general
relativistic effects in the strong-field regime.
