QED symmetries in real-time thermal field theory
  We study the discrete and gauge symmetries of Quantum Electrodynamics at finite temperature within the real-time formalism.   The gauge invariance of the complete generating functional leads to the finite temperature Ward identities. These Ward identities relate the eight vertex functions to the elements of the self-energy matrix. Combining the relations obtained from the $Z_2$ and the gauge symmetries of the theory we find that only one out of eight longitudinal vertex functions is independent. As a consequence of the Ward identities it is shown that some elements of the vertex function are singular when the photon momentum goes to zero. 
Juan Carlos D'Olivo, Manuel Torres and Eduardo Tututi