Isospin violation in low-energy pion-nucleon scattering revisited

We calculate isospin breaking in pion-nucleon scattering in the threshold region in the framework of covariant baryon chiral perturbation theory. All effects due to quark mass differences as well as real and virtual photons are consistently included. As an application, we discuss the energy dependence of the triangle relation that connects elastic scattering on the proton pi± p --> pi± p with the charge exchange reaction pi- p --> pi0 n.