Radio ejection in the evolution of X-ray binaries: the bridge between\n low mass X-ray binaries and millisecond pulsars

We present a scenario for the spin-up and evolution of binary millisecond\npulsars. This can explain the observational properties of the recently\ndiscovered binary millisecond pulsar PSR J1740-5340, with orbital period 32.5\nhrs, in the Globular Cluster NGC 6397. The optical counterpart of this system\nis a star as luminous as the cluster turnoff stars, but with a lower Teff (a\nlarger radius) which we model with a star of initial mass compatible with the\nmasses evolving in the cluster (~0.85 Msun). This star has suffered Roche lobe\noverflow while evolving off the main sequence, spinning up the neutron star to\nthe present period of 3.65 ms. There are evidences that at present, Roche lobe\noverflow is still going on. Indeed Roche lobe deformation of the mass losing\ncomponent is necessary to be compatible with the optical light curve. The\npresence of matter around the system is also consistent with the long lasting\nirregular radio eclipses seen in the system. We propose that this system is\npresently in a phase of `radio-ejection' mass loss. The radio-ejection phase\ncan be initiated only if the system is subject to intermittency in the mass\ntransfer during the spin-up phase. In fact, when the system is detached the\npulsar radio emission is not quenched, and may be able to prevent further mass\naccretion due to the action of the pulsar pressure at the inner Lagrangian\npoint.\n