PSR J1518+4904: A mildly relativistic binary pulsar system

PSR J1518+4904 is a recently discovered 40.9 ms pulsar in an 8.6 day, moderately eccentric orbit. We have measured pulse arrival times for this pulsar over 1.4 yr at several radio frequencies, from which we have derived high-precision rotational, astrometric, and orbital parameters. The upper limit for the period derivative of the pulsar, P < 4 x 10(-20), gives a characteristic age of at least 1.6 x 10(10) yr, which is among the highest known. We find the orbit to be precessing at a rate of 0 degrees.0111 +/- 0 degrees.0002 yr(-1), which yields a total system mass (pulsar plus companion) of 2.62 +/- 0.07 M(.) according to general relativity. Further analysis of the orbital parameters yields a firm upper limit of 1.75 M(.) on the pulsar mass and constrains the companion mass to the range 0.9-2.7 M(.). These masses, together with the sizable orbital eccentricity and other evidence, strongly suggest that the companion is a second neutron star.