We report the discovery by the RXTE PCA of a second transient accreting millisecond pulsar , XTE J1751–305 , during regular monitoring observations of the galactic bulge region .
The pulsar has a spin frequency of 435 Hz , making it one of the fastest pulsars .
The pulsations contain the signature of orbital Doppler modulation , which implies an orbital period of 42 minutes , the shortest orbital period of any known radio or X-ray millisecond pulsar .
The mass function , f _ { x } = ( 1.278 \pm 0.003 ) \times 10 ^ { -6 } M _ { \sun } , yields a minimum mass for the companion of between 0.013 and 0.017 M _ { \sun } , depending on the mass of the neutron star .
No eclipses were detected .
A previous X-ray outburst in June , 1998 , was discovered in archival All-Sky Monitor data .
Assuming mass transfer in this binary system is driven by gravitational radiation , we constrain the orbital inclination to be in the range 30°–85° , and the companion mass to be 0.013–0.035 M _ { \sun } .
The companion is most likely a heated helium dwarf .
We also present results from the Chandra HRC-S observations which provide the best known position of XTE J1751–305 .