  We analyze the Fubini-Furlan-Rosetti sum rule in the framework of covariant
baryon chiral perturbation theory to leading one-loop accuracy and including
next-to-leading order polynomial contributions. We discuss the relation between
the subtraction constants in the invariant amplitudes and certain low-energy
constants employed in earlier chiral perturbation theory studies of threshold
neutral pion photoproduction off nucleons. In particular, we consider the
corrections to the sum rule due to the finite pion mass and show that below the
threshold they agree well with determinations based on fixed-t dispersion
relations. We also discuss the energy dependence of the electric dipole
amplitude E_{0+}.

Nuclear Theory