Context : PSR J1518 + 4904 is one of only 9 known double neutron star systems .
These systems are highly valuable for measuring the masses of neutron stars , measuring the effects of gravity , and testing gravitational theories .

Aims : We determine an improved timing solution for a mildly relativistic double neutron star system , combining data from multiple telescopes .
We set better constraints on relativistic parameters and the separate masses of the system , and discuss the evolution of PSR J1518 + 4904 in the context of other double neutron star systems .

Methods : PSR J1518 + 4904 has been regularly observed for more than 10 years by the European Pulsar Timing Array ( EPTA ) network using the Westerbork , Jodrell Bank , Effelsberg and Nançay radio telescopes .
The data were analysed using the updated timing software tempo2 .

Results : We have improved the timing solution for this double neutron star system .
The periastron advance has been refined and a significant detection of proper motion is presented .
It is not likely that more post-Keplerian parameters , with which the individual neutron star masses and the inclination angle of the system can be determined separately , can be measured in the near future .

Conclusions : Using a combination of the high-quality data sets present in the EPTA collaboration , extended with the original GBT data , we have constrained the masses in the system to m _ { \mathrm { p } } < 1.17 M _ { \odot } and m _ { \mathrm { c } } > 1.55 M _ { \odot } ( 95.4 \% confidence ) , and the inclination angle of the orbit to be less than 47 degrees ( 99 % ) .
From this we derive that the pulsar in this system possibly has one of the lowest neutron star masses measured to date .
From evolutionary considerations it seems likely that the companion star , despite its high mass , was formed in an electron-capture supernova .