  Relativistic phenomena (orbital precession, Shapiro delay, and/or orbital
decay) have been measured in Arecibo timing observations of four pulsar-white
dwarf binaries, leading to constraints on the neutron star masses. We have
detected the decay of the PSR J0751+1807 orbit due to gravitational radiation
emission, the first such measurement in a binary with a low mass ratio
(m_2/m_1<<1). Timing data constrains the mass of this pulsar to bet between 1.6
and 2.8 solar masses. Masses of the other pulsars are in marginal agreement
with the canonical pulsar mass of 1.35 solar masses, but higher values are
preferred.
