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A gradual rotation of the direction of periapsis within the orbital plane. A closed Kepler orbit has fixed argument of periapsis $\omega$; relativistic gravity can make $\omega$ advance between successive orbits. The angle change and binary-pulsar orbital decay describe different changes of the orbit.

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]