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The surprise was a pulsar whose apparent period would not stay fixed. Russell Alan Hulse's Arecibo search used a multichannel receiver, pulsar dedispersion and computer searches over period and pulse width. Follow-up at higher time resolution retained the changing period, excluding inadequate sampling as its explanation. The observed variation exceeded the expected terrestrial Doppler shift; a predicted reversal from decreasing to increasing period appeared on 16 September 1974.^[ref]

The 1975 analysis fitted an eccentric Kepler orbit to Doppler shifts inferred from about 200 five-minute period measurements over ten days. That discovery fit was not yet the later precision arrival-time experiment.^[ref] An orbit explains both signs of the apparent period change without requiring the stellar rotation to alternate between speeding up and slowing down.

Joseph Hooton Taylor Jr. and Russell Alan Hulse discovered a pulsar in a binary star system. Changes in the timing of the pulses revealed the pulsar's orbital motion, making an otherwise unseen compact companion measurable.^[ref]

Long-term pulse timing then tested general relativity. After accounting for other timing effects, the decreasing orbital period agreed with the loss of orbital energy predicted from gravitational radiation. The system supplied an indirect test of gravitational waves through their effect on an orbiting source.^[ref]

Awarded in equal shares to Russell Alan Hulse and Joseph Hooton Taylor Jr. for discovering a new type of pulsar that opened new possibilities for studying gravity.^[ref]

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]

Russell Alan Hulse discovered a pulsar whose measured pulse period changed systematically during repeated observations. With Joseph Hooton Taylor Jr., he interpreted the variations as orbital Doppler shifts in a compact binary star system.

The changing period was the clue rather than a defect in the clock. A regular intrinsic pulse train observed from an orbiting source provides a radial velocity curve and orbital timing information. The discovery created a laboratory for later tests of relativistic gravity, including the loss of orbital energy through gravitational radiation.^[ref][ref]