Invariances of regularized QED diagrams
  We consider properties of connected diagrams with fermion-photon interaction and such fermion and photon propagators and vertex function that the values of these diagrams are finite. We establish the properties of these propagators and vertex function that imply that these diagrams are invariant under C, P, T, CP, CT, PT, or CPT transformations up to some phase factor common to each process. We introduce eight new transformations related to Hermiticity and establish the conditions under which they leave the tree transition probabilities invariant. We determine such general Lorentz form-invariant fermion and photon propagators and fermion-photon vertex functions that make diagrams Lorentz-invariant. 
Marijan Ribaric and Luka Sustersic