  We report the discovery of burst oscillations at the spin frequency in ten
thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR
J17511-3057. The burst oscillation properties are, like those from the
persistent AMXPs SAX J1808.4-3658 and XTE J1814-338, anomalous compared to
burst oscillations from intermittent pulsars or non-pulsing LMXBs. Like SAX
J1808.4-3658 they show frequency drifts in the rising phase rather than the
tail. There is also evidence for harmonic content. Where IGR J17511-3057 is
unusual compared to the other two persistent pulsars is that oscillations are
not detected throughout all bursts. As accretion rate drops the bursts get
brighter and their rise/decay time scales become shorter, while the oscillation
amplitude falls below the detection threshold: first in the burst peak and then
also in the rise. None of the bursts from IGR J17511-3057 show evidence for
photospheric radius expansion (which might be expected to suppress oscillation
amplitude) which allow us to set an upper limit to the distance of 6.9 kpc. We
discuss the implications of our results for models of the burst oscillation
mechanism.
