  SAX J1810.8-2609 is a faint X-ray transient, mostly known for its abnormally
low quiescent thermal luminosity, which disagrees with standard cooling models.
It is also one of a small sample of stars whose mass and radius have been
estimated using spectral modeling of one of its thermonuclear bursts. Here we
report the discovery of millisecond oscillations in a type I thermonuclear
X-ray burst from SAX J1810.8-2609 observed by RXTE during the 2007 outburst. A
strong signal (Leahy-normalized power of 71.5, 4.5e-9 chance of coincidence
with a conservative estimate for the number of trials) was present at 531.8 Hz
during the decay of one out of six bursts observed. Oscillations were detected
for about 6 seconds, during which their frequency increased from 531.4 to 531.9
Hz in a manner similar to other burst oscillation sources. The millisecond
oscillations discovered pinpoint the spin frequency of the neutron star, which
is important for the spectral modeling, associated mass-radius inference, and
understanding the evolutionary status and cooling behavior of the star. As of
April 2018 the source is in outburst again, providing a fleeting opportunity to
acquire new material for the burst oscillation searches.
