The Orbit and Companion of Probable gamma-ray Pulsar J2339-0533

We have measured dramatic flux and spectral variations through the 0.193d orbit of the optical counterpart of the unidentified gamma-ray source 0FGL J2339.8-0530. This compact object companion is strongly heated, with T_eff varying from ~6900K (superior conjunction) to <3000K at minimum. A combined fit to the light curve and radial velocity amplitudes implies M_1 ~ 0.075M_sun, M_2 ~ 1.4 M_sun and inclination i ~ 57deg. Thus this is a likely `black widow' system with Edot~10^{34-34.5} erg/s pulsar driving companion mass loss. This wind, also suggested by the X-ray light curve, may prevent radio pulse detection. Our measurements constrain the pulsar's reflex motion, increasing the possibility of a pulse detection in the gamma-ray signal.