The recently discovered1 1.6 ms binary radio pulsar PSR1957 + 20 shows radio eclipses whose duration indicates that the occulting body is substantially larger than the Roche lobe of the low-mass (~10-2Msolar) companion star. This suggests1-4 that this companion is evaporating through the action of a strong pulsar energy flux5. An optical counterpart has been detected which shows brightness variations in phase with the 9.2 h orbital cycle6. We have obtained optical charge coupled device (CCD) images which show that the counterpart is one component of a close visual pair separated by ~0.7 arcsec. At maximum both are equally bright with combined V magnitude of 19.9, while at minimum PSR1957 + 20 is invisible. From spectroscopic observations we find that the contaminating star is a normal G star. The spectrum of PSR1957 + 20 shows intermittent Hα emission. We confirm that the optical brightness of PSR1957 + 20 varies in phase with the radio Doppler velocity curve1, and find that the amplitude is probably more than 3 magnitudes, minimum light coinciding with the radio eclipse. The optical light curve is consistent with heating of a hydrogen-rich low-mass white dwarf by high-energy radiation from the nearby millisecond pulsar.