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

MILLISECOND pulsars are usually found in binary systems. This is in ke eping with the generally accepted model for the formation of such puls ars14 in which an old neutron star is spun up to high angular velociti es by the accretion of matter from a companion star. The millisecond p ulsar 1620-26 in the globular cluster M4 is no exception: timing measu rements1,2 reveal the presence of a 0.3-solar-mass companion star (pro bably a white dwarf) with an orbital period of 191 days. But subsequen t measurements of this pulsar have identified a small but significant deviation from the expected behaviour3,4, suggestive of an unusually l arge second derivative in the pulsar's rotation rate5. Here we examine several possible sources-both intrinsic and extrinsic-for this deriva tive, and we find that it is best explained by the presence of a secon d, weakly bound companion object moving in a wide orbit around the mai n binary system. The third body in this hierarchical system has an orb ital period of approximately 100 years and a mass approximately ten ti mes that of Jupiter, and may have been captured during a recent collis ion with another stellar system6.