  Isolated hot subdwarfs might be formed by the merging of two helium-core
white dwarfs. Before merging, helium-core white dwarfs have hydrogen-rich
envelopes and some of this hydrogen may survive the merger. We calculate the
mass of hydrogen that is present at the start of such mergers and, with the
assumption that hydrogen is mixed throughout the disrupted white dwarf in the
merger process, estimate how much can survive. We find a hydrogen mass of up to
about $2 \times 10^{-3}\,\mathrm{M}_{\odot}$ in merger remnants. We make model
merger remnants that include the hydrogen mass appropriate to their total mass
and compare their atmospheric parameters with a sample of apparently isolated
hot subdwarfs, hydrogen-rich sdBs. The majority of these stars can be explained
as the remnants of double helium white dwarf mergers.
