  We present new results based on observations carried out with the Rossi X-ray
Timing Explorer during the decay of an outburst of the low-mass X-ray binary
(LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in
4U 1608-52, one of the long-standing issues about the phenomenology of the
kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique
relation between the frequency of the kHz QPOs and the X-ray flux. We show that
despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs
is monotonically related to the position of the source in the color-color
diagram. Our findings strengthen the idea that, as in the case of Z sources, in
the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that
the observed changes in the frequency of the kHz QPOs in LMXBs are driven by
changes in $\dot M$. These results raise some concern about the recently
reported detection of the orbital frequency at the innermost stable orbit in 4U
1820-30.
