  We consider the interpretation of a pair of kHz Quasi Periodic Oscillations
(QPOs) in the Fourier spectra of two Low Mass X-Ray Binaries, Sco X-1 and
4U1608-52, hosting an old accreting neutron star. The observed frequency
difference of these QPOs decreaseas as their frequency increases, contrary to
simple beat frequency models, which predict a constant frequency difference. We
show that the behaviour of these QPOs is instead well matched in terms of the
fundamental frequencies (in the radial and azimuthal directions) for test
particle motion in the gravitational field of the neutron star, for reasonable
star masses, and nearly independent of the star spin. The radial frequency must
be much smaller than the azimuthal one, testifying that kHz QPOs are produced
close to the innermost stable orbit. These results are not reproduced through
the post--Newtonian (PN) approximation of General Relativity (GR). kHz QPOs
from X-ray binaries likely provide an accurate laboratory for strong field GR.
