A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. In at least one case, the double pulsar PSR J0737-3039, the companion star is another pulsar as well.
Binary pulsars are one of the few objects which allow physicists to test general relativity in the case of a strong gravitational field. Although the binary companion to the pulsar is usually difficult or impossible to observe, the timing of the pulses from the pulsar can be measured with extraordinary accuracy by radio telescopes. Binary pulsar timing has indirectly confirmed the existence of gravitational radiation and verified Einstein's general theory of relativity.