Spectroscopic Analysis of the Companion to the Binary MSP PSR J1740‐5340 in NGC 6397

By means of high‐resolution spectra, we have measured radial velocities of the companion (hereafter COM J1740‐5340) to the eclipsing millisecond pulsar PSR J1740‐5340 in the galactic globular cluster NGC 6397. The radial velocity curve enables us to derive the mass ratio (MPSR/MCOM = 5.85 ± 0.13).The derived abundances are fully compatible with those of normal unperturbed stars in NGC 6397, with the exception of a few elements. The lack of C, in particular, suggests that the star has been peeled down to regions where incomplete CNO burning occurs, supporting a scenario where the companion is a turn‐off star which has lost most of its mass.A detailed analysis of the Hα lines reveals that optically thin hydrogen gas resided outside the Roche lobe of COM J1740‐5340. The line morphology suggests the presence of a steam of material going from the companion toward the neutron star; this material never reachs the neutron star surface, being driven back by the pulsar radiation, far beyond COM J1740‐5340.The unexp...