A millisecond pulsar in an eclipsing binary

We have discovered a remarkable pulsar with period 1.6 ms, moving in a nearly circular 9.17-h orbit around a low-mass companion star. At an observing frequency of 430 MHz, the pulsar, PSR1957 + 20, is eclipsed once each orbit for about 50 minutes. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds—presumably as a result of propagation through plasma surrounding the companion. The pulsar's orbit about the system barycentre has a radius of 0.089 light seconds projected on to the line of sight. The observed orbital period and size, together with the fact that eclipses occur, imply a surprisingly low companion mass, only a few per cent of the mass of the Sun.