THE MASS OF THE COMPANION OF PSR-1718-19

The recently-discovered binary pulsar 1718 - 19 displays periodic obsc uration. We show that free-free absorption in a spherical outflow from the companion star reproduces the observed light curves for a system inclination of order 35-degrees. This implies a companion of mass cong ruent-to 0.2 M.; if it is close to the main sequence this star is well inside its Roche lobe. The spindown torque in the wind could account for the large pulsar period derivative without the need to invoke a ma gnetic field which is unusually strong for a globular cluster pulsar. The stellar wind of a detached companion provides a natural source for the obscuring matter. This and the derived companion mass tend to sup port the capture formation scenario favored by Wijers & Paczynski (199 3). However, it is also possible that the system descended from a low- mass X-ray binary which ceased mass transfer at an orbital period of a bout 3 hr, and has since been widened by adiabatic expansion of the co mpanion through pulsar ablation.