  Observations using the Rossi X-ray Timing Explorer have discovered dozens of
accreting neutron stars with millisecond spin periods in low-mass binary star
systems. Eighteen are millisecond X-ray pulsars powered by accretion or nuclear
burning or both. These stars have magnetic fields strong enough for them to
become millisecond rotation-powered (radio) pulsars when accretion ceases. Few,
if any, accretion- or rotation-powered pulsars have spin rates higher than 750
Hz. There is strong evidence that the spin-up of some accreting neutron stars
is limited by magnetic spin equilibrium whereas the spin-up of others is halted
when accretion ends. Further study will show whether the spin rates of some
accretion- or rotation-powered pulsars are or were limited by emission of
gravitational radiation.
