Gravitationally bound three-body systems have been studied for hundreds of years1, 2 and are common in our Galaxy3, 4. They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies5 and test theories of gravity6, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7, 8; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13)
, where
is the solar mass and the parentheses contain the uncertainty in the
final decimal places) and the two white dwarf companions (0.19751(15)
and 0.4101(3)
), as well as the inclinations of the orbits (both about 39.2°). The
unexpectedly coplanar and nearly circular orbits indicate a complex and
exotic evolutionary past that differs from those of known stellar
systems. The gravitational field of the outer white dwarf strongly
accelerates the inner binary containing the neutron star, and the system
will thus provide an ideal laboratory in which to test the strong
equivalence principle of general relativity.
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