  If the inflaton is a pseudo-scalar axion, the axion shift symmetry can
protect the flatness of its potential from too large radiative corrections.
This possibility, known as natural inflation, requires an axion scale which is
greater than the (reduced) Planck scale. It is unclear whether such a high
value is compatible with an effective field theoretical description, and if the
global axionic symmetry survives quantum gravity effects. We propose a
mechanism which provides an effective large axion scale, although the original
one is sub-Planckian. The mechanism is based on the presence of two axions,
with a potential provided by two anomalous gauge groups. The effective large
axion scale is due to an almost exact symmetry between the couplings of the
axions to the anomalous groups. We also comment on a possible implementation in
heterotic string theory.
