The light curve of the companion to PSR J2051-0827

We have obtained new photometry of the companion to the eclipsing binary mi llisecond pulsar PSR J2051-0827. The data are modeled by a gravitationally distorted, low-mass secondary star that is irradiated by the impinging puls ar wind. The best-fit models require that greater than 30% of the incident energy is absorbed by the secondary star and reradiated as optical emission . Unless the companion is significantly farther away than its dispersion me asure distance, it underfills its Roche lobe by at least 30%. A small compa nion makes it difficult to drive a wind from its surface and thus explain t he mass loss in the system. Our models show that the system is inclined by more than 30 degrees, and thus the companion mass is in the range of 0.025- 0.055 M. (for a 1.4 M. pulsar). The unilluminated side of the companion is cool, with a best-fit temperature of 3000 K.