  I summarize the available RXTE data of the 4 low-mass X-ray binaries (LMXBs)
and Atoll sources Aql X-1, 4U 1728-34, 4U 1608-52, and 4U 1636-53. I
concentrate on the relation between the frequencies of the quasi-periodic
oscillations at kilohertz frequencies (kHz QPOs) and the X-ray flux and colors
of these sources. In these 4 sources the kHz QPOs are only observed in a narrow
range of spectral states (as defined from the X-ray color-color diagrams). I
show that despite its complex dependence upon the X-ray flux, the frequency of
the kHz QPOs is monotonically related to the position of the source in the
color-color diagram. These findings strengthen the idea that in LMXBs the X-ray
flux is not a good indicator of the mass accretion rate, $\dot M$, and that the
observed changes in the frequency of the kHz QPOs in LMXBs are driven by
changes in $\dot M$.
