Virtual photons in baryon chiral perturbation theory

We construct the general Lagrangian for relativistic and heavy baryon chira l perturbation theory with virtual photons to fourth order. We work out the electromagnetic and strong isospin violating contributions to the nucleon self-energy, the nucleon mass and the scalar form factor of the nucleon. El ectromagnetic effects for the shift to the Cheng-Dashen point can be as lar ge as 2 MeV. We also discuss the corrections to Weinberg's prediction for t he scattering length difference a(pi(0)p) - a(pi(0)n) and show that they ar e small. (C) 1999 Elsevier Science B.V. All rights reserved.