Discovery of the Millisecond X-Ray Pulsar HETE J1900.1-2455

We report the discovery of millisecond pulsations from the low-mass X-ray binary HETE J1900.1-2455, which was discovered by the detection of a type I X-ray burst by the High Energy Transient Explorer 2 (HETE-2). The neutron star emits coherent pulsations at 377.3 Hz and is in an 83.3 minute circular orbit with a companion of mass greater than 0.016 M_solar and likely less than 0.07 M_solar. The companion star's Roche lobe could be filled by a brown dwarf with no need for heating or nonstandard evolution. During one interval with an unusually high X-ray flux, the source produced quasi-periodic oscillations with a single peak at 883 Hz, and on subsequent days the pulsations were suppressed. We consider the distribution of spin versus orbital period in neutron star low-mass X-ray binaries.