We test the relativistic precession model ( RPM ) and the MHD Alfvén 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 \pm 0.10 M _ { \odot } ( 2.83 \pm 0.04 M _ { \odot } and 1.85 \pm 0.02 M _ { \odot } ) , which is \sim 30 \% ( 100 % and 40 % ) higher than the measured result 1.5 \pm 0.3 M _ { \odot } ( < 1.4 M _ { \odot } and 1.29 ^ { +0.19 } _ { -0.07 } M _ { \odot } ) .
For AWOM , where the free parameter of model is the density of star , we infer the NS radii to be around 10 \sim 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 .