We present results of Rossi X-Ray Timing Explorer observations of the
low-mass X-ray binary and atoll source 4U 1608-52 made over 9 days dur
ing the decline of an X-ray intensity outburst in 1996 March. A fast-t
iming analysis shows a strong and narrow quasi-periodic oscillation (Q
PO) peak at frequencies between 850 and 890 Hz on March 3 and 6, as we
ll as a broad peak around 690 Hz on March 9. Observations on March 12
show no significant signal. On March 3, the X-ray spectrum of the QPO
is quite hard; its strength increases steadily from 5% at similar to 2
keV to similar to 20% at similar to 12 keV. The QPO frequency varies
between 850 and 890 Hz on that day, and the peak widens and its rms de
creases with centroid frequency in a way very similar to the well-know
n horizontal branch oscillations (HBO) in Z sources. We apply the HBO
beat frequency model to atoll sources and suggest that, whereas the mo
del could produce QPOs at the observed frequencies, the lack of correl
ation we observe between QPO properties and X-ray count rate is hard t
o reconcile with this model.