  [Abridged] Type-I X-ray bursts are thermonuclear flashes that take place on
the surface of accreting neutron stars. The wait time between consecutive
bursts is set by the time required to accumulate the fuel needed to trigger a
new burst; this is at least one hour. Sometimes secondary bursts are observed,
approximately 10 min after the main burst. These short wait-time bursts are not
yet understood. We observed the low-mass X-ray binary and X-ray burster EXO
0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting
up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts
occur in burst triplets, with wait times of 12 min between the three components
of the triplet. We also detect 14 doublets with similar wait times between the
two components of the doublet. The characteristics of the bursts indicate that
possibly all bursts in this system are hydrogen-ignited, in contrast with most
other frequent X-ray bursters in which bursts are helium-ignited, but
consistent with the low mass accretion rate in EXO 0748-676. Possibly the
hydrogen ignition is the determining factor for the occurrence of short
wait-time bursts.
