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.
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Hankins, T. H., Stinebring, D. R. & Rawley, L. A. Astrophys. J. 315, 149–153 (1987).
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Fruchter, A., Stinebring, D. & Taylor, J. A millisecond pulsar in an eclipsing binary. Nature 333, 237–239 (1988). https://doi.org/10.1038/333237a0
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