  Twin peak high frequency quasi-periodic oscillations (HF QPOs) observed in
the power spectra of Low Mass X-ray Binaries (LMXBs), with either a black hole
or a neutron star, have central frequencies that are typical of the orbital
motion time-scale close to the compact object. Thus, twin HF QPOs might carry
the fingerprint of physical effects in a strongly curved space-time. We study
the azimuth phase \phi(t) for orbital motion in the Schwarzschild metric and
calculate the power spectra to check whether they display the features seen in
the observed ones. We show that the timing of \phi(t) on non-closed orbits can
account for the observed twin peak HF QPOs. The uppermost couple of peaks in
frequency has the lower peak that corresponds to the azimuthal frequency
\nu_{\phi}, the upper one to \nu_{\phi}+\nu_r. The azimuth phase temporal
behavior \phi(t) on a slightly eccentric orbit in the Schwarzschild metric is
described by a linear function of slope \nu_{\phi} plus an oscillating term at
the relativistic radial frequency \nu_r. We deduce that the twin peak HF QPOs
might originate from a frequency modulated (FM) signal driven by the kinematics
of orbital motion in a curved space-time.
