  The role of recombination during a common-envelope event has been long
debated. Many studies have argued that much of hydrogen recombination energy,
which is radiated in relatively cool and optically-thin layers, might not
thermalise in the envelope. On the other hand, helium recombination contains
30% of the total recombination energy, and occurs much deeper in the stellar
envelope. We investigate the distinct roles played by hydrogen and helium
recombination in a common-envelope interaction experienced by a 12 solar mass
red supergiant donor. We perform adiabatic, 3D hydrodynamical simulations that
(i) include hydrogen, helium, and molecular hydrogen recombination, (ii)
include hydrogen and helium recombination, (iii) include only helium
recombination, and (iv) do not include recombination energy. By comparing these
simulations, we find that the addition of helium recombination energy alone
ejects 30% more envelope mass, and leads to a 16% larger post-plunge-in
separation. Under the adiabatic assumption, adding hydrogen recombination
energy increases the amount of ejected mass by a further 40%, possibly
unbinding the entire envelope, but does not affect the post-plunge separation.
Most of the ejecta becomes unbound at relatively high (>70%) degrees of
hydrogen ionisation, where the hydrogen recombination energy is likely to
expand the envelope instead of being radiated away.
