  We present new Rossi X-ray Timing Explorer observations of the low-mass X-ray
binary 4U 1608-52 during the decay of its 1998 outburst. We detect by a direct
FFT method the existence of a second kilohertz quasi-periodic oscillation (kHz
QPO) in its power density spectrum, previously only seen by means of the
sensitivity-enhancing `shift and add' technique. This result confirms that 4U
1608-52 is a twin kHz QPO source. The frequency separation between these two
QPO decreased significantly, from 325.5 +/- 3.4 Hz to 225.3 +/- 12.0 Hz, as the
frequency of the lower kHz QPO increased from 470 Hz to 865 Hz, in
contradiction with a simple beat-frequency interpretation. This change in the
peak separation of the kHz QPOs is closely similar to that previously seen in
Sco X-1, but takes place at a ten times lower average luminosity. We discuss
this result within the framework of models that have been proposed for kHz QPO.
Beat frequency models where the peak separation is identified with the neutron
star spin rate, as well as the explanations previously proposed to account for
the similar behavior of the QPOs in Sco X-1, are strongly challenged by this
result.
