The orbital light curve of PSR 1957 + 20

We make a detailed study of the optical light curve of PSR 1957 + 20. We show that the deep, smooth, and symmetrical modulation can be successfully modeled by a highly irradiated secondary. Two types of models are consistent with the data: (1) a secondary close to filling its Roche lobe with 10%-20% of the incident flux converted to optical emission, and (2) a secondary considerably underfilling its Roche lobe, with a high degree of beaming of the neutron star flux. However, the second model can be rejected on the basis of current estimates of the extinction toward 1957 + 20. The proximity of the photosphere of the secondary to its Roche lobe facilitates mass loss by irradiation. Our ignorance of the (nonirradiated) luminosity of the secondary allows us to place only weak constraints on the inclination - 50°-80°. There is no evidence in the light curve for optical emission from any source in PSR 1957 + 20 other than the secondary itself.