  The axion photon system in an external magnetic field, when for example
considered with the geometry of the experiments exploring axion photon mixing,
displays a continuous axion-photon duality symmetry in the limit the axion mass
is neglected. The conservation law that follows from this symmetry is obtained.
The magnetic field interaction is seen to be equivalent to first order to the
interaction of a complex charged field with an external electric potential,
where this ficticious "electric potential" is proportional to the external
magnetic field. This allows to solve for the scattering amplitudes using
already known scalar QED results. It is argued that in more generic conditions
(not just related to these experiments) axion-photon condensation could be
obtained for high magnetic fields. Finally an exact constraint originating
fromthe current conservation on the amplitudes of reflected and transmited
waves is obtained.
