  Common envelope (CE) phases in binary systems where the primary star reaches
the tip of the red giant branch are discussed as a formation scenario for hot
subluminous B-type (sdB) stars. For some of these objects, observations point
to very low-mass companions. In hydrodynamical CE simulations with the
moving-mesh code AREPO, we test whether low-mass objects can successfully
unbind the envelope. The success of envelope removal in our simulations
critically depends on whether or not the ionization energy released by
recombination processes in the expanding material is taken into account. If
this energy is thermalized locally, envelope ejection eventually leading to the
formation of an sdB star is possible with companion masses down to the brown
dwarf range. For even lower companion masses approaching the regime of giant
planets, however, envelope removal becomes increasingly difficult or impossible
to achieve. Our results are consistent with current observational constraints
on companion masses of sdB stars. Based on a semianalytic model, we suggest a
new criterion for the lowest companion mass that is capable of triggering a
dynamical response of the primary star thus potentially facilitating the
ejection of a common envelope. This gives an estimate consistent with the
findings of our hydrodynamical simulations.
