  Fast spinning neutron stars, recycled in low mass binaries, may have accreted
a substantial amount of mass. The available relativistic measurements of
neutron star masses, all clustering around 1.4 M_sun, however refer mostly to
slowly rotating neutron stars which accreted a tiny amount of mass during
evolution in a massive binary system. We develop a semi-analytical model for
studying the evolution of the spin period P of a magnetic neutron star as a
function of the baryonic mass load M_{ac}; evolution is followed down to
submillisecond periods and the magnetic field is allowed to decay significantly
before the end of recycling. We use different equations of state and include
rotational deformation effects, the presence of a strong gravitational field
and of a magnetosphere. For the non-magnetic case, comparison with numerical
relativistic codes shows the accuracy of our description. The minimum accreted
mass requested to spin-up a magnetized 1.35M_sun-neutron star at a few
millisecond is 0.05 M_sun, while this value doubles for an unmagnetized neutron
star. Below 1 millisecond the request is of at least 0.25 M_sun. There may
exist a yet undetected population of massive submillisecond neutron stars. The
discovery of a submillisecond neutron star would imply a lower limit for its
mass of about 1.7M_sun.
