  We report the discovery of a quasi-periodic oscillation (QPO) at 61.0+/-1.7
Hz with the Rossi X-Ray Timing Explorer in the low-mass X-ray binary and
persistently bright atoll source GX 13+1 (4U 1811-17). The QPO had an rms
amplitude of 1.7+/-0.2 % (2-13.0 keV) and a FWHM of 15.9+/-4.2 Hz. Its
frequency increased with count rate and its amplitude increased with photon
energy. In addition a peaked noise component was found with a cut-off frequency
around 2 Hz, a power law index of around -4, and an rms amplitude of ~1.8 %,
probably the well known atoll source high frequency noise. It was only found
when the QPO was detected. Very low frequency noise was present with a power
law index of ~1, and an rms amplitude of ~4 %. A second observation showed
similar variability components. In the X-ray color-color diagram the source did
not trace out the usual banana branch, but showed a two branched structure.
  This is the first detection of a QPO in one of the four persistently bright
atoll sources in the galactic bulge. We argue that the QPO properties indicate
that it is the same phenomenon as the horizontal branch oscillations (HBO) in Z
sources. That HBO might turn up in the persistently bright atoll sources was
previously suggested on the basis of the magnetospheric beat frequency model
for HBO. We discuss the properties of the new phenomenon within the framework
of this model.
