Optical observations of the binary pulsar system PSR B1718-19: Implications for tidal circularization

We report on Keck and Hubble Space Telescope optical observations of the ec lipsing binary pulsar system PSR B1718-19, in the direction of the globular cluster NGC 6342. These reveal a faint star (m(F702W) = 25.21 +/- 0.07; Ve ga system) within the pulsar's 0 ".15 radius positional error circle. This may be the companion. If it is a main-sequence star in the cluster, it has radius R-C similar or equal to 0.3 R., temperature T-eff similar or equal t o 3600 K, and mass M-C similar or equal to 0.3 M.. In many formation models , however, the pulsar (spun-up by accretion or newly formed) and its compan ion are initially in an eccentric orbit. If so, for tidal circularization t o have produced the present-day highly circular orbit, a large stellar radi us is required, i.e., the star must be bloated. Using constraints on the ra dius and temperature from the Roche and Hayashi limits, we infer from our o bservations that R-C less than or similar to 0.44 R. and T-eff greater than or similar to 3300 K. Even for the largest radii, the required efficiency of tidal dissipation is larger than expected for some prescriptions.