  By using the stellar evolution code MESA we show that most of the hydrogen
recombination energy that is released as the envelope expands during a regular
common envelope evolution (CEE), namely, the initial dynamical phase or
plunge-in phase, is radiated, and hence increases substantially the stellar
luminosity. Only about ten per cent of the hydrogen recombination energy might
be used to remove the envelope. We show that the key property of energy
transport is that when convection becomes inefficient in the outer parts of the
envelope, where the ionization degree of hydrogen falls below about 30 per
cent, photon diffusion becomes very efficient and removes the recombination
energy. The expanding envelope absorbs most of the gravitational energy that is
released by the spiraling-in process of the secondary star inside the common
envelope, and so it is the hydrogen recombination energy that is behind most of
the luminosity increase of the system. The recombination energy of hydrogen
adds only a small fraction of the energy required to remove the common
envelope, and hence does not play a significant role in the ejection of the
envelope.

Solar and Stellar Astrophysics