  Millisecond radio pulsars are neutron stars that have been spun-up by the
transfer of angular momentum during the low-mass X-ray binary phase. The
transition from an accretion-powered to a rotation-powered pulsar takes place
on evolutionary timescales at the end of the accretion process, however it may
also occur sporadically in systems undergoing transient X-ray activity. We have
obtained the first optical spectrum of the low mass transient X-ray pulsar SAX
J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar
B1957+20, this X-ray pulsar shows a large optical modulation at the orbital
period due to an irradiated companion star. Using the brightness of the
companion star as a bolometer, we conclude that a very high irradiating
luminosity, a factor of ~100 larger than directly observed, must be present in
the system. This most likely derives from a rotation-powered neutron star that
resumes activity during quiescence.
