  Starting from the observation that kilohertz Quasi Period Oscillations (kHz
QPO) occur in a very narrow range of X-ray luminosities in neutron star low
mass X-ray binaries, we try to link the kHz QPO observability to variations of
the neutron star magnetospheric radius, in response to changing mass inflow
rate. At low luminosities, the drop off of kHz QPOs activity may be explained
by the onset of the centrifugal barrier, when the magnetospheric radius reaches
the corotation radius. At the opposite side, at higher luminosities, the
magnetospheric radius may reach the neutron star and the vanishing of the
magnetosphere may led to the stopping of the kHz QPOs activity. If we apply
these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1)
and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits
naturally apply in the framework of beat frequency models but can also work in
the case of general relativistic models.
