THE BINARY PULSAR PSRJ-2145-0750 - A SYSTEM ORIGINATING FROM A LOW ORINTERMEDIATE-MASS X-RAY BINARY WITH A DONOR STAR ON THE ASYMPTOTIC GIANT BRANCH

It is shown that the 6.84-day orbital period and relatively high compa nion mass of the 16.1ms period binary pulsar PSRJ 2145-0750 find a con sistent explanation if the progenitor-system consisted of a donor star in the mass range 1 - 6 M. with an orbital period such that this star overflowed its Roche-lobe on the Asymptotic Giant Branch (AGB). The p resent system then resulted after a phase of Common Envelope (CE) evol ution. Initial donor masses in the range 2.4 - 6 M. are possible only if, in addition to the orbital gravitational binding energy, other ene rgy sources are available for expelling the envelope. The post-CE remn ants of donors in the mass range 4 - 6 M. overflow their Roche lobes a nd continue to transfer mass at a near or super Eddington rate for of order 10(6) yrs. This provides a simple way to explain the rapid spin and weak magnetic held of PSRJ 2145-0750.