A 2ND COMPANION OF THE MILLISECOND PULSAR-1620-26

MILLISECOND pulsars are usually found in binary systems. This is in keeping with the generally accepted model for the formation of such pulsars14 in which an old neutron star is spun up to high angular velocities by the accretion of matter from a companion star. The millisecond pulsar 1620-26 in the globular cluster M4 is no exception: timing measurements1,2 reveal the presence of a 0.3-solar-mass companion star (probably a white dwarf) with an orbital period of 191 days. But subsequent measurements of this pulsar have identified a small but significant deviation from the expected behaviour3,4, suggestive of an unusually large second derivative in the pulsar's rotation rate5. Here we examine several possible sources-both intrinsic and extrinsic-for this derivative, and we find that it is best explained by the presence of a second, weakly bound companion object moving in a wide orbit around the main binary system. The third body in this hierarchical system has an orbital period of approximately 100 years and a mass approximately ten times that of Jupiter, and may have been captured during a recent collision with another stellar system6.