  An X-ray monitoring campaign with Chandra and Swift confirms that 1RXS
J171824.2-402934 is accreting at the lowest rate among the known persistently
accreting low-mass X-ray binaries. A thermonuclear X-ray burst was detected
with the all-sky monitor on RXTE. This is only the second such burst seen in
1RXS J171824.2-402934 in more than 20 Ms of observations done over 19 years.
The low burst recurrence rate is in line with the low accretion rate. The
persistent nature and low accretion rate can be reconciled within accretion
disk theory if the binary system is ultracompact. An unprecedentedly short
orbital period of less than approximately 7 minutes would be implied. An
ultracompact nature, together with the properties of the type I X-ray burst,
suggests, in turn, that helium-rich material is accreted. Optical follow-up of
the Chandra error region does not reveal an unambiguous counterpart.
