  We describe observations of the seventh accretion-powered millisecond pulsar,
HETE J1900.1-2455 made with the Rossi X-ray Timing Explorer during the year of
activity that followed its discovery in 2005 June. We detected intermittent
pulsations at a peak fractional amplitude of 3%, but only in the first two
months of the outburst. On three occasions during this time we observed an
abrupt increase in the pulse amplitude, approximately coincident with the time
of a thermonuclear burst, followed by a steady decrease on a timescale of
approx. 10 d. HETE J1900.1-2455 has shown the longest active period by far for
any transient accretion-powered millisecond pulsar, comparable instead to the
outburst cycles for other transient X-ray binaries. Since the last detection of
pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion
rate, non-pulsing LMXB; we hypothesize that other, presently active LMXBs may
have also been detectable initially as millisecond X-ray pulsars.
