PSR J1518- A MILDLY RELATIVISTIC BINARY PULSAR SYSTEM(4904 )

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 t his pulsar over 1.4 yr at several radio frequencies, from which we hav e derived high-precision rotational, astrometric, and orbital paramete rs. The upper limit for the period derivative of the pulsar, P < 4 x 1 0(-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 ra te of 0 degrees.0111 +/- 0 degrees.0002 yr(-1), which yields a total s ystem 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.