AXION-ELECTROMAGNETIC WAVES

We extend the duality symmetry between the electric and the magnetic fields to the case in which an additional axion-like term is present, and we derive the set of Maxwell equations that preserves this symmetry. This new set of equations allows for a gauge symmetry extending the ordinary symmetry, in the classical electrodynamics. We obtain explicit solutions for the new set of equations in the absence of external sources, and we discuss the implications of a new internal symmetry between the axion field and the electromagnetic gauge potential.