The triple pulsar system PSR B1620-26 in M4

The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a whit e dwarf companion in a half-year orbit. Anomalously large variations in the pulsar's apparent spin-down rate have suggested the presence of a second c ompanion in a much wider orbit. Using timing observations made on more than 700 days spanning 11 yr, we confirm this anomalous timing behavior, We exp licitly demonstrate, for the first time, that a timing model consisting of the sum of two noninteracting Keplerian orbits can account for the observed signal. Both circular and elliptical orbits are allowed, although highly e ccentric orbits require improbable orbital geometries. The motion of the pu lsar in the inner orbit is very nearly a Keplerian ellipse, but the tidal e ffects of the outer companion cause variations in the orbital elements. We have measured the change in the projected semimajor axis of the orbit, whic h is dominated by precession-driven changes in the orbital inclination. Thi s measurement, along with limits on the rate of change of other orbital ele ments, can be used to significantly restrict the properties of the outer or bit. We find that the second companion most likely has a mass m similar to 0.01 M.-it is almost certainly below the hydrogen-burning limit (m < 0.036 M., 95% confidence)-and has a current distance from the binary of similar t o 35 AU and orbital period of order 100 yr. Circular (and near-circular) or bits are allowed only if the pulsar magnetic field is similar to 3 x 10(9) G, an order of magnitude higher than a typical millisecond pulsar field str ength. In this case, the companion has mass m similar to 1.2 x 10(-3) M. an d orbital period similar to 62 yr.