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 Δ Pb/Pb = 1.6 × 10⁻⁷, changing quadratically with time and displaying an orbital period second derivative Pb = (1.43 ± 0.08) × 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 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 0.025\,M_ companion to PSR~B1957+20 is most likely non-degenerate, convective, and magnetically active.