  Aims. We derive simple estimates of the maximum efficiency with which matter
can be ejected by the propeller mechanism in disk-fed, rotating magnetic
neutron stars. Some binary evolution scenarios envisage that this mechanism is
responsible for expelling to infinity the mass inflowing at a low rate from the
companion star, therefore limiting the total amount of mass that can be
accreted by the neutron star. Methods. We demonstrate that, for typical neutron
star parameters, a maximum of \eta_{pro} < 5.7 (P_{-3})^{1/3} times more matter
than accreted can be expelled through the propeller mechanism at the expenses
of the neutron star rotational energy (P_{-3} is the NS spin period in unit of
10E-3 s). Approaching this value, however, would require a great deal of fine
tuning in the system parameters and the properties of the interaction of matter
and magnetic field at the magnetospheric boundary. Results. We conclude that
some other mechanism must be invoked in order to prevent that too much mass
accretes onto the neutron stars of some low mass X-ray binaries.
