  Gas-phase sulphur bearing volatiles appear to be severely depleted in
protoplanetary disks. The detection of CS and non-detections of SO and SO2 in
many disks have shown that the gas in the warm molecular layer, where giant
planets accrete their atmospheres, has a high C/O ratio. In this letter, we
report the detection of SO and SO2 in the Oph-IRS 48 disk using ALMA. This is
the first case of prominent SO2 emission detected from a protoplanetary disk.
The molecular emissions of both molecules is spatially correlated with the
asymmetric dust trap. We propose that this is due to the sublimation of ices at
the edge of the dust cavity and that the bulk of the ice reservoir is
coincident with the millimetre dust grains. Depending on the partition of
elemental sulphur between refractory and volatile materials the observed
molecules can account for 15-100% of the total sulphur budget in the disk. In
strong contrast to previous results, we constrain the C/O ratio from the CS/SO
ratio to be < 1 and potentially solar. This has important implications for the
elemental composition of planets forming within the cavities of warm transition
disks.
