  We study in a systematic way the quality factor of the lower and upper kHz
QPOs in a sample of low luminosity neutron star X-ray binaries, showing both
QPOs varying over a wide frequency range. The sample includes 4U 1636-536, 4U
1608-522, 4U 1735-44, 4U 1728-34, 4U 1820-303 and 4U 0614+09. We find that all
sources except 4U 0614+091 show evidence of a drop in the quality factor of
their lower kHz QPOs at high frequency. For 4U 0614+091 only the rising part of
the quality factor versus frequency curve has been sampled so far. At the same
time, in all sources but 4U 1728-34, the quality factor of the upper kilo-Hz
QPO increases all the way to the highest detectable frequencies. We show that
the high-frequency behaviours of both the lower and upper kHz QPO quality
factors are consistent with what is expected if the drop is produced by the
approach of an active oscillating region to the innermost stable circular
orbit: the existence of which is a key feature of General Relativity in the
strong field regime. Within this interpretation, our results imply
gravitational masses around 2 solar masses for the neutron stars in those
systems.
