  A handful of white dwarfs with helium-dominated atmospheres contain
exceptionally large masses of hydrogen in their convection zones, with the
metal-polluted white dwarf GD 16 being one of the earliest recognised examples.
We report the discovery of a similar star: the white dwarf coincidentally named
GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and
calculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe and
Ni. The metal abundance ratios indicate that the two stars recently accreted
debris which is Mg-poor compared to the composition of bulk Earth. However,
unlike the metal pollutants, H never diffuses out of the atmosphere of white
dwarfs and we propose that the exceptionally high atmospheric H content of GD
16 and GD 17 ($2.2\times 10^{24}$g and $2.9\times 10^{24}$g respectively) could
result from previous accretion of water bearing planetesimals. Comparing the
detection of trace H and metal pollution among 729 helium atmosphere white
dwarfs, we find that the presence of H is nearly twice as common in
metal-polluted white dwarfs compared to their metal-free counterparts. This
highly significant correlation indicates that, over the cooling age of the
white 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 the
most plausible source is water. In this scenario, water must be common in
systems with rocky planetesimals.
