Compact double neutron star (DNS) systems are unique laboratories for testing gravitational theories and studying DNS mergers. Here we report the properties of a new DNS system, PSR J1856–0039, discovered in the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulsar is mildly recycled with a period of 23.4 ms in a compact eccentric orbit ($e=0.106$) with an orbital period of 2.36 hours. By following up FAST observations, we measured the relativistic effects, including the orbital period derivative ${\dot{P}}_{\mathrm{orb}}=-1.284\pm 0.019\times {10}^{-12}\mathrm{s}{\mathrm{s}}^{-1}$, periastron advance $\dot{\omega }=17.5859\pm 0.0007\mathrm{deg}{\mathrm{yr}}^{-1}$, and Einstein delay $\gamma =0.445\pm 0.011\mathrm{ms}$. This DNS system has a low orbital inclination of $i=133\stackrel{\circ }{.}2\pm 1\stackrel{\circ }{.}1$ and the lowest total mass of any known DNS, $M_{\mathrm{tot}}=2.48841\pm 0.00015M_{\odot }$, with a determined pulsar mass of $1.304\pm 0.022M_{\odot }$ and a companion mass of $1.185\pm 0.022M_{\odot }$, one of the lowest neutron-star masses. The observed orbital decay due to gravitational-wave emission ${\dot{P}}_{\mathrm{orb},\mathrm{obs}}^{\mathrm{GW}}$ and the orbital decay predicted by general relativity ${\dot{P}}_{\mathrm{orb},\mathrm{pred}}^{\mathrm{GW}}$ are consistent at a level of ${\dot{P}}_{\mathrm{orb},\mathrm{obs}}^{\mathrm{GW}}/{\dot{P}}_{\mathrm{orb},\mathrm{pred}}^{\mathrm{GW}}=1.009(14)$ (68% confidence). This DNS will merge after 82 Myr and may form a stable neutron star or collapse into a black hole after spin-down. Long-term monitoring could potentially probe the Lense-Thirring precession.

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• Astrophysical studies of gravity
• General relativity
• Neutron stars & pulsars