A binary pulsar in a 53-minute orbit.

Spider pulsars are neutron stars that have a companion star in a close orbit. The companion star sheds material to the neutron star, spinning it up to millisecond rotation periods, while the orbit shortens to hours. The companion is eventually ablated and destroyed by the pulsar wind and radiation. Spider pulsars are key for studying the evolutionary link between accreting X-ray pulsars and isolated millisecond pulsars, pulsar irradiation effects and the birth of massive neutron stars. Black widow pulsars in extremely compact orbits (as short as 62 minutes) have companions with masses much smaller than 0.1 M. They may have evolved from redback pulsars with companion masses of about 0.1-0.4 M and orbital periods of less than 1 day. If this is true, then there should be a population of millisecond pulsars with moderate-mass companions and very short orbital periods, but, hitherto, no such system was known. Here we report radio observations of the binary millisecond pulsar PSR J1953+1844 (M71E) that show it to have an orbital period of 53.3 minutes and a companion with a mass of around 0.07 M. It is a faint X-ray source and located 2.5 arcminutes from the centre of the globular cluster M71.