The discovery of a second millisecond binary radio pulsar1, PSR1855+09 (period P = 5ms), with a relatively wide circular orbit and low mass function indicates that the incidence of such systems (see also refs 2, 3) among the galactic radio pulsar population is ∼4 × ≳10-3. The generally accepted model for the origin of these systems is that they have descended from low-mass X-ray binaries in which a weakly magnetized neutron star3 was spun-up by the accretion of matter from an evolved low-mass (≲1.2 M⊙) companion star, which has now ended its life as a helium white dwarf3-8. We show here that, based on the incidence of the progenitor low-mass X-ray binaries (LMXBs), the observed incidence of millisecond binary radio pulsars can be explained only if, at their present low field strength of ∼109G, the decay timescale of neutron-star surface magnetic dipole moments is 322153a0109 yr. This possibility has been advanced by Kulkarni9, who estimates, based on the optical detection of the counterpart of the weak-field binary radio pulsar PSR0655 + 64, that this pulsar is old (>5 × 108 yr) and hence that the magnetic field decay timescale becomes very large (>109 yr) for field strengths ≲1010 G. Our findings provide independent support for this possibility.
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