Generalized Dirac duality and CP violation in a two photon theory
  A kinetic mixing term, which generalizes the duality symmetry of Dirac, is studied in a theory with two photons (visible and hidden). This theory can be either CP conserving or CP violating depending on the transformation of fields in the hidden sector. However if CP is violated, it necessarily occurs in the hidden sector. This opens up an interesting possibility of new sources of CP violation. 
P. Arias, A. K. Das, J. Gamboa and F. Mendez
hep-ph hep-th