  We observed the accreting millisecond pulsar SAX J1808.4-3658 with
Gemini-South in g' and i' bands, nearly simultaneous with XMM-Newton
observations. A clear periodic flux modulation on the system's orbital period
is present, consistent with the varying aspect of the donor star's heated face.
We model the contributions of a disk and donor star to these optical bands. To
produce the observed modulation amplitudes, we conclude that the donor must be
irradiated by an external flux 2 orders of magnitude greater than provided by
the measured X-ray luminosity. A possible explanation for this irradiation is
that the radio pulsar mechanism becomes active during the quiescent state as
suggested by Burderi et al., with relativistic particles heating the donor's
day-side face. Our modelling constrains the binary inclination to be between 36
and 67 degrees. We obtain estimates for the pulsar mass of >2.2 solar masses
(although this limit is sensitive to the source's distance), consistent with
the accelerated NS cooling in this system indicated by X-ray observations. We
also estimate the donor mass to be in the range of 0.07-0.11 solar masses,
providing further indications that the system underwent non-standard binary
evolution to reach its current state.
