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A handful of white dwarfs with helium-dominated atmospheres containexceptionally large masses of hydrogen in their convection zones, with themetal-polluted white dwarf GD 16 being one of the earliest recognised examples.We report the discovery of a similar star: the white dwarf coincidentally namedGD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 andcalculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe andNi. The metal abundance ratios indicate that the two stars recently accreteddebris which is Mg-poor compared to the composition of bulk Earth. However,unlike the metal pollutants, H never diffuses out of the atmosphere of whitedwarfs and we propose that the exceptionally high atmospheric H content of GD16 and GD 17 ($2.2\times 10^{24}$g and $2.9\times 10^{24}$g respectively) couldresult from previous accretion of water bearing planetesimals. Comparing thedetection of trace H and metal pollution among 729 helium atmosphere whitedwarfs, we find that the presence of H is nearly twice as common inmetal-polluted white dwarfs compared to their metal-free counterparts. Thishighly significant correlation indicates that, over the cooling age of thewhite dwarfs, at least some fraction of the H detected in many He atmospheres(including GD 16 and GD 17) is accreted alongside metal pollutants, where themost plausible source is water. In this scenario, water must be common insystems with rocky planetesimals.

Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion