Radio observations of binary pulsars: Clues to binary evolution and tests of general relativity

We have carried out pulse timing experiments on nine radio pulsars in binary systems to obtain refined rotational, astrometric, and orbital information. We discuss the implications of our results for theories of binary evolution and relativistic gravity.; We have detected non-monotonic orbital period changes in the PSR B1957+20 system similar to those observed 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 variable mass loss from the companion's surface, which is heated by radiation from the pulsar. These observations challenge existing scenarios of the companion's destruction in a system believed to be the evolutionary link between low-mass X-ray binaries and isolated millisecond pulsars.; Binary pulsars are believed to be ideal laboratories for studying the nature of the gravitational interaction in the strong-field regime. A number of systems serve as useful testing grounds for the Strong Equivalence Principle: we set improved bounds on the existence of dipolar gravitational radiation, time-variability of the universal constant of gravitation G, and the differential acceleration of strongly self-gravitating objects in an external gravitational field. Within the framework of general relativity, our measurements of the periastron advance rates in the PSR B1802{dollar}-{dollar}07 and B2303+46 systems yield the total mass of each system and constrain the individual masses of their components--such mass measurements constrain both models of binary evolution in high-mass systems and theories of neutron star structure. We update timing results for the relativistic binary pulsar B1534+12, and present polarization measurements which describe the shape and geometry of its radio beam. We detect a gradual long-term change in the pulse shape of PSR B1534+12, evidence for relativistic precession of the pulsar spin axis.