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I describe recent timing and polarimetric observations of a number of binary pulsar systems with implications for theories of binary evolution and relativistic gravity.
In the PSR~B1957+20 system, we have detected non-secular orbital period changes similar to those seen in other close binaries but previously unknown in a pulsar system; such orbital variability may imply that the pulsar's low-mass companion is magnetically active or may be the result of a varying mass-loss rate or emission geometry from the companion's surface as it is heated by radiation from the pulsar. In the systems containing PSRs B1802$-$07 and B2303+46, new and refined measurements, respectively, of the rate of periastron advance have yielded within the framework of General Relativity the total mass of each system and constrained the individual masses of their components. High time-resolution polarimetry of PSR B1534+12 has provided the geometry of the pulsar radio beam, constraining the spin-orbital misalignment of the visible neutron star; we also describe a study of pulse shape variability for this pulsar as a means of detecting orbital aberration and relativistic precession of the pulsar spin axis. We have refined the spin, astrometric, and orbital parameters of PSRs B0655+64, B0820+02, and B1820$-$11, and improved the limits on dipolar gravitational radiation, violation of the Strong Equivalence Principle, and time-variability of the universal constant of gravitation $G$ attainable with these timing experiments.
