SAX J1808.4-3658
A transient X-ray source first discovered in 1996 by the Italian BeppoSAX satellite, SAX J1808.4-3658 revealed X-ray pulsations at the 401 Hz neutron star spin frequency when it was observed during a subsequent outburst in 1998 by NASA's RXTE satellite. The neutron star orbits a brown dwarf binary companion every 2.01 hours.
These pulsars are thought to be the evolutionary progenitors of recycled millsecond pulsars. A total of six accreting millisecond X-ray pulsars have been discovered as of February 2005.