Photon helicity and quantum anomalies in curved spacetimes

We compare several definitions of photon helicity (magnetic, electric, electromagnetic) present in literature. In curved spacetime quantum anomalies can spoil helicity conservation inducing, according to these definitions, different effects on photons: either rotation of linear polarization or production of circular polarization. We derive the Noether current associated with duality transformations starting from manifestly invariant Lagrangians.