Discovery of Two Relativistic Neutron Star-White Dwarf Binaries

We have discovered two recycled pulsars in relativistic orbits as part of the first high-frequency survey of intermediate Galactic latitudes. PSR J1157-5112 is a 44 ms pulsar and the first recycled pulsar with an ultramassive (M > 1.14 M☉) white dwarf companion. Millisecond pulsar J1757-5322 is a relativistic circular-orbit system that will coalesce because of the emission of gravitational radiation in less than 9.5 Gyr. Of the ~40 known circular-orbit pulsars, J1757-5322 and J1157-5112 have the highest projected orbital velocities. There are now three local neutron star/white dwarf binaries that will coalesce in less than a Hubble time, implying a large coalescence rate for these objects in the local universe. Systems such as J1141-6545 are potential gamma-ray burst progenitors and dominate the coalescence rate, while lighter systems make excellent progenitors of millisecond pulsars with planetary or ultra-low mass companions.