  We test the relativistic precession model (RPM) and the MHD Alfven wave
oscillation model (AWOM) for the kHz QPOs by the sources with measured NS
masses and twin kHz QPO frequencies. For RPM, the derived NS mass of Cyg X-2
(SAX J1808.4-3658 and 4U 1820-30) is 1.96 +/- 0.10 solar masses (2.83 +/- 0.04
solar masses and 1.85 +/- 0.02 solar masses), which is 30% (100% and 40%)
higher than the measured result 1.5 +/- 0.3 solar masses (< 1.4 solar masses
and 1.29 + 0.19 / - 0.07 solar masses). For AWOM, where the free parameter of
model is the density of star, we infer the NS radii to be around 10 - 20 km for
the above three sources, based on which we can infer the matter compositions
inside NSs with the help of the equations of state (EOSs). In particular, for
SAX J1808.4-3658, AWOM shows a lower mass density of its NS than those of the
other known kHz QPO sources, with the radius range of 17 - 20 km, which
excludes the strange quark matter inside its star.
