  The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object
between accreting X-ray binaries and millisecond radio pulsars. We have
constrained the thermal radiation from its surface through XMM-Newton X-ray
observations, providing strong evidence for neutrino cooling processes from the
neutron star core. We have also undertaken simultaneous X-ray and optical
(Gemini) observations, shedding light on whether the strong heating of the
companion star in quiescence may be due to X-ray irradiation, or to a radio
pulsar turning on when accretion stops.
