DETECTION OF AN IRRADIATED PULSAR COMPANION

We have detected the optical companion of the eclipsing millisecond pu lsar J2051 - 0827. The amplitude of the companion's light curve is at least 1.2 mag, and the variation is consistent with the companion's ro tating synchronously about the pulsar and one side being heated by the impinging pulsar flux. PSR J2051-0827 is only the second pulsar binar y for which direct heating of the companion has been observed. The R m agnitude at maximum brightness is 22.3, but we failed to detect any fl ux from the companion at the phase corresponding to radio eclipse. The absolute magnitude and color temperature suggest that the companion f ills its Roche lobe.