2 v 1 1 5 D ec 1 99 3 Orbital Variability in the Eclipsing Pulsar Binary PSR B 1957 + 20

We have conducted timing observations of the eclipsing millisecond binary pulsar PSR B1957+20, extending the span of data on this pulsar to more than five years. During this time the orbital period of the system has varied by roughly ∆Pb/Pb = 1.6 × 10, changing quadratically with time and displaying an orbital period second derivative P̈b = (1.43±0.08)×10 −18 s. The previous measurement of a large negative orbital period derivative reflected only the short-term behavior of the system during the early observations; the orbital period derivative is now positive and increasing rapidly. If, as we suspect, the PSR B1957+20 system is undergoing quasi-cyclic orbital period variations similar to those found in other close binaries such as Algol and RS CVn, then the 0.025M⊙ companion to PSR B1957+20 is most likely non-degenerate, convective, and magnetically active. Subject headings: pulsars — binaries: evolution — stars: eclipsing binaries — stars: individual (PSR B1957+20)