The orbital period derivative of PSR 1957+20

In light of recent observations of PSR 1957+20, we suggest a mechanism in which the mass loss by the secondary carries away varying degrees of specific angular momentum. We present a model which incorporates Roche‐lobe overflow and wind driven mass loss. In our model, Roche‐lobe overflow of the companion produces an evaporating disk or ring. An external disk around the binary may also be created by the mass loss. Changes in the structure of the accretion disk and external disk produce variations in the specific angular momentum carried away by the mass loss. These variations are likely to occur over a viscous timescale. This mechanism could account for the dramatic changes of Pb observed and predicts alternating cycles of expansion and contraction of the binary orbit.