  We re-examine the correlation between the frequencies of upper and lower kHz
quasi-periodic oscillations (QPO) in bright neutron-star low-mass X-ray
binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and
two accreting millisecond pulsars in our sample, we show that the full sample
does not support the class of theoretical models based on a single resonance,
while models based on relativistic precession or Alfven waves describe the data
better. Moreover, we show that the fact that all sources follow roughly the
same correlation over a finite frequency range creates a correlation between
the linear parameters of the fits to any sub-sample.
