  We present results of RXTE observations of the low-mass X-ray binary and
atoll source 4U 1608$-$52 made over 9 days during the decline of an X-ray
intensity outburst in March 1996. A fast-timing analysis shows a strong and
narrow quasi periodic oscillation (QPO) peak at frequencies between 850 and 890
Hz on March 3 and 6, and 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 $\sim$2 to $\sim$20
\% at $\sim$12~keV. The QPO frequency varies between 850 and 890 Hz on that
day, and the peak widens and its rms decreases with centroid frequency in a way
very similar to the well-known horizontal branch oscillations (HBO) in
Z-sources. We apply the HBO beat frequency model to atoll sources, and suggest
that, whereas the model could produce QPOs at the observed frequencies, the
lack of correlation we observe between QPO properties and X-ray count rate is
hard to reconcile with this model.
