ORBITAL VARIABILITY IN THE ECLIPSING PULSAR BINARY PSR B1957+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 DELTAP(b)/P(b) = 1.6 x 10(-7), changing quadratically with time and displaying an orbital period second derivative P(b) = (1.43 +/- 0.08) x 10(-18) s-1. 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. 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 0.025 M. companion to PSR B1957 + 20 is most likely non-degenerate, convective, and magnetically active.