The four known binary radio pulsars (Table 1) seem to fall into two different categories. Two of them, PSR0655 + 64 and PSR1913 + 16, have short orbital periods (<25 h)1,2and high mass functions, indicating companion masses >0.8 M (respectively, about 1 M and 1.4 M (ref. 2)). The other two, PSR0820 + 02 and PSR1953 + 29, have long orbital periods ( 120 days), nearly circular orbits and low almost identical mass functions of ∼3 × 10-3 M, suggesting companion masses of ∼0.2-0.4 M (refs 3-5). We point out here that these two classes of systems are expected to be formed by the later evolution of binaries consisting of a neutron star and a normal companion star, in which the companion was (considerably) more massive than the neutron star, or less massive than the neutron star, respectively. Mass transfer from an evolved companion that is more massive than the neutron star (more precisely, mass ratio ≥0.85 (refs 6, 7)) tends to be unstable and to lead to runaway mass transfer and spiral-in resulting in a very short orbital period, whereas mass transfer from an evolved companion that is less massive than the neutron star is stable and leads to expansion of the orbit 6,7. Furthermore, we point out that in the systems with a less massive companion, the neutron star most probably was formed by the accretion-induced collapse of a white dwarf. Such a model explains in a natural way why PSR1953 + 29 has a millisecond rotation period and why PSR0820 + 02 has not.
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