  Using archival RXTE data, we show that the ultracompact X-ray binary in NGC
1851 exhibits large amplitude X-ray flux varations of more than a factor of 10
on timescales of days to weeks and undergoes sustained periods of months where
the time-averaged luminosty varies by factors of two. Variations of this
magnitude and timescale have not been reported previously in other ultracompact
X-ray binaries. Mass transfer in ultracompact binaries is thought to be driven
by gravitational radiation and the predicted transfer rates are so high that
the disks of ultracompact binaries with orbits as short as that of this object
should not be susceptible to ionization instabilities. Therefore the
variability characteristics we observe were unexpected, and need to be
understood. We briefly discuss a few alternatives for producing the observed
variations in light of the fact that the viscous timescale of the disk is of
order a week, comparable to the shorter time scale variation that is observed
but much less than the longer term variation. We also discuss the implications
for interpretation of observations of extragalactic binaries if the type of
variability seen in the source in NGC 1851 is typical.
