  WD J204713.76-125908.9 is a new addition to the small class of white dwarfs
with helium-dominated photospheres that exhibit strong Balmer absorption lines
and atmospheric metal pollution. The exceptional abundances of hydrogen
observed in these stars may be the result of accretion of water-rich rocky
bodies. We obtained far-ultraviolet and optical spectroscopy of WD
J204713.76-125908.9 using the Cosmic Origin Spectrograph on-board the Hubble
Space Telescope and X-shooter on the Very Large Telescope, and identify
photospheric absorption lines of nine metals: C, O, Mg, Si, P, S, Ca, Fe and
Ni. The abundance ratios are consistent with the steady state accretion of
exo-planetesimal debris rich in the volatile elements carbon and oxygen, and
the transitional element sulphur, by factors of seventeen, two, and four
respectively compared to bulk Earth. The parent body has a composition akin to
Solar 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 the
composition of the disrupted parent body, finding from our simulations a median
water mass fraction of eight per cent.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics