Accurate mass ratio and heating effects in the dual-line millisecond binary pulsar 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 fully confirms that COM J1740-5340 is orbiting the pulsar, and this enables us to derive the most accurate mass ratio (M-PSR/M-COM = 5.85 +/- 0.13) for any nonrelativistic binary system containing a neutron star. Assuming a pulsar mass in the range 1.3-1.9 M-., the mass of COM J1740-5340 spans the interval 0.22-0.32 M-., the inclination of the system is constrained within, and the Roche lobe 56degrees greater than or similar to i greater than or similar to 47degrees and the Roche lobe radius is r(RL) similar to1.5-1.7 R-.. A preliminary chemical abundance analysis confirms that COM J1740-5340 has a metallicity compatible with that measured for other stars in this metal-poor globular, but the unexpected detection of strong He I absorption lines implies the existence of regions at K, which are significantly warmer T > 10,000 than the rest of the star. The intensity of this line correlates with the orbital phase, suggesting the presence of a region on the companion surface, heated by the millisecond pulsar flux.