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WD J204713.76-125908.9 is a new addition to the small class of white dwarfswith helium-dominated photospheres that exhibit strong Balmer absorption linesand atmospheric metal pollution. The exceptional abundances of hydrogenobserved in these stars may be the result of accretion of water-rich rockybodies. We obtained far-ultraviolet and optical spectroscopy of WDJ204713.76-125908.9 using the Cosmic Origin Spectrograph on-board the HubbleSpace Telescope and X-shooter on the Very Large Telescope, and identifyphotospheric absorption lines of nine metals: C, O, Mg, Si, P, S, Ca, Fe andNi. The abundance ratios are consistent with the steady state accretion ofexo-planetesimal debris rich in the volatile elements carbon and oxygen, andthe transitional element sulphur, by factors of seventeen, two, and fourrespectively compared to bulk Earth. The parent body has a composition akin toSolar System carbonaceous chondrites, and the inferred minimum mass, $1.6\times 10^{20}$ g, is comparable to an asteroid 23 km in radius. We model thecomposition of the disrupted parent body, finding from our simulations a medianwater mass fraction of eight per cent.

White dwarf pollution by hydrated planetary remnants: Hydrogen and Metals in WD J204713.76-125908.9