  We report on the discovery by the Swift Gamma-Ray Burst Explorer of the
eighth known transient accretion-powered millisecond pulsar, SWIFT
J1756.9-2508, as part of routine observations with the Swift Burst Alert
Telescope hard X-ray transient monitor. The pulsar was subsequently observed by
both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer
Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an
orbital period of 54.7 minutes. The minimum companion mass is between 0.0067
and 0.0086 solar masses, depending on the mass of the neutron star, and the
upper limit on the mass is 0.030 solar masses (95% confidence level). Such a
low mass is inconsistent with brown dwarf models, and comparison with white
dwarf models suggests that the companion is a He-dominated donor whose thermal
cooling has been at least modestly slowed by irradiation from the accretion
flux. No X-ray bursts, dips, eclipses or quasi-periodic oscillations were
detected. The current outburst lasted approximately 13 days and no earlier
outbursts were found in archival data.
