  We have continued our long term study of the double-neutron-star binary
pulsar PSR B1534+12, using new instrumentation to make very high precision
measurements at the Arecibo Observatory. We have significantly improved our
solution for the astrometric, spin, and orbital parameters of the system, as
well as for the five "post-Keplerian" orbital parameters that can be used to
test gravitation theory. The results are in good agreement with the predictions
of general relativity.
  With the assumption that general relativity is the correct theory of gravity
in the classical regime, our measurements allow us to determine the masses of
the pulsar and its companion neutron star with high accuracy: 1.3332 \pm 0.0010
Msun and 1.3452 \pm 0.0010Msun, respectively. The small but significant mass
difference is difficult to understand in most evolutionary models, as the
pulsar is thought to have been born first from a more massive progenitor star
and then undergone a period of mass accretion before the formation of the
second neutron star.
  PSR B1534+12 has also become a valuable probe of the local interstellar
medium. We have now measured the pulsar distance to be 1.02 \pm 0.05 kpc,
giving a mean electron density along this line of sight of 0.011/cc. We
continue to measure a gradient in the dispersion measure, though the rate of
change is now slower than in the first years after the pulsar's discovery.
