  The cool white dwarf SDSS J124231.07+522626.6 exhibits photospheric
absorption lines of 8 distinct heavy elements in medium resolution optical
spectra, notably including oxygen. The Teff = 13000 K atmosphere is
helium-dominated, but the convection zone contains significant amounts of
hydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, and
Fe) account for almost all the accreted mass, totalling at least 1.2e+24 g,
similar to the mass of Ceres. The time-averaged accretion rate is 2e+10 g/s,
one of the highest rates inferred among all known metal-polluted white dwarfs.
We note a large oxygen excess, with respect to the most common metal oxides,
suggesting that the white dwarf accreted planetary debris with a water content
of ~38 per cent by mass. This star, together with GD 61, GD 16, and GD 362,
form a small group of outliers from the known population of evolved planetary
systems accreting predominantly dry, rocky debris. This result strengthens the
hypothesis that, integrated over the cooling ages of white dwarfs, accretion of
water-rich debris from disrupted planetesimals may significantly contribute to
the build-up of trace hydrogen observed in a large fraction of helium-dominated
white dwarf atmospheres.
