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 M _ { \odot } and 1.3452 \pm 0.0010 M _ { \odot } , 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 cm ^ { -3 } .
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 .