Dependence of the frequency of the kilohertz quasi-periodic oscillations on X-ray count rate and colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-m y Timing Explorer during the decay of an outburst of the low-mass X-ray bin ary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at l east in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely the lack o f a unique relation between the frequency of the kHz QPOs and the X-ray flu x. We show that despite its complex dependence on the X-ray flux, the frequ ency 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 t he case of Z sources, in the atoll sources the X-ray flux is not a good ind icator of M and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in (M) over dot. These results raise some co ncern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.