  Neutron stars are the densest objects known in the Universe. Being the final
product of stellar evolution, their internal composition and structure is
rather poorly constrained by measurements.
  It is the purpose of this paper to put some constrains on the mass and moment
of inertia of neutron stars based on the interpretation of kHz quasi-periodic
oscillations observed in low mass X-ray binaries.
  We use observations of high-frequency quasi-periodic observations (HF-QPOs)
in low mass X-ray binaries (LMXBs) to look for the average mass and moment of
inertia of neutron stars. This is done by applying our parametric resonance
model to discriminate between slow and fast rotators.
  We fit our model to data from ten LMXBs for which HF-QPOs have been seen and
the spin of the enclosed accreting neutron star is known. For a simplified
analysis we assume that all neutron stars possess the same properties (same
mass $M_*$ and same moment of inertia $I_*$). We find an average mass $M_*
\approx 2.0-2.2\, M_{\odot}$. The corresponding average moment of inertia is
then $I_* \approx 1-3 \times 10^{38}\;{\rm kg\,m^2} \approx 0.5-1.5 \,
(10\;\textrm{ km})^2 \, M_\odot$ which equals to dimensionless spin parameter
$\tilde{a} \approx 0.05-0.15$ for slow rotators (neutron stars with a spin
frequency roughly about 300~Hz) respectively $\tilde{a} \approx 0.1-0.3$ for
fast rotators (neutron stars with the spin frequency roughly about 600~Hz).
