An eclipsing millisecond pulsar with a possible main-sequence companion inNGC 6397

We present the results of 1 yr of pulse timing observations of PSR J1740-53 40, an eclipsing millisecond pulsar located in the globular cluster NGC 639 7. We have obtained detailed orbital parameters and a precise position for the pulsar. The radio pulsar signal shows frequent interactions with the at mosphere of the companion and suffers significant and strongly variable del ays and intensity variations over a wide range of orbital phases. These cha racteristics and the binary parameters indicate that the companion may be a bloated main-sequence star or the remnant (still overflowing its Roche lob e) of the star that spun up the pulsar. In both cases, this would be the fi rst binary millisecond pulsar system known with such a companion.