The optical counterpart of the binary millisecond X-ray pulsar SAX J 1808.4-3658 during quiescence was detected at V = 21.5 mag, inconsistent with intrinsic emission from the fain companion star. We propose that the optical emission from this system during quiescence is due to the irradiation of the companion star and a remnant accretion disk by the rotational energy released by the fast spinning neutron star, switched on, as magneto-dipole rotator (radio pulsar). In this scenario the companion behaves as a bolometer, reprocessing in optical part of the power emitted by the pulsar. The reprocessed fraction depends only on known binary parameters. Thus the blackbody temperature of the companion can be predicted and compared with the observations. Our computations indicate that the observed optical magnitudes are fully consistent with this hypothesis. In this case the observed optical luminosity may be the first evidence that a radio pulsar is active in this system in quiescence.
