  PSR J0751+1807 is a millisecond pulsar in a circular 6 hr binary system with
a helium white dwarf secondary. Through high precision pulse timing
measurements with the Arecibo and Effelsberg radio telescopes, we have detected
the decay of its orbit due to emission of gravitational radiation. This is the
first detection of the relativistic orbital decay of a low-mass, circular
binary pulsar system. The measured rate of change in orbital period, corrected
for acceleration biases, is dP_b/dt=(-6.4+-0.9)x10^-14. Interpreted in the
context of general relativity, and combined with measurement of Shapiro delay,
it implies a pulsar mass of 2.1+-0.2 solar masses, the most massive pulsar
measured. This adds to the emerging trend toward relatively high neutron star
masses in neutron star--white dwarf binaries. Additionally, there is some
evidence for an inverse correlation between pulsar mass and orbital period in
these systems. We consider alternatives to the general relativistic analysis of
the data, and we use the pulsar timing data to place limits on violations of
the strong equivalence principle.
