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A binary pulsar is a pulsar in a binary star: the companion's gravity changes the emitting neutron star's motion and hence its pulse arrival times. The companion need not itself be a detectable pulsar. Pulsar timing turns that motion into a measurable orbit, making the system useful for stellar mass determination and tests of general relativity.

Russell Alan Hulse and Joseph Hooton Taylor Jr. discovered this binary pulsar at Arecibo in 1974; their January 1975 paper reports a roughly $59\mathrm{ms}$ pulse period and $7.75\mathrm{h}$ orbital period. Its unseen companion was not detected as a second radio pulsar. Discovery of the orbit preceded the later binary-pulsar orbital decay test.^[ref]

The 1993 Nobel Prize in Physics recognized the discovery.^[ref]

By 1993, the system was interpreted as a pair of neutron stars.^[ref]