  We investigate the dispersive contribution by radiative processes such as
(pi- proton to neutron gamma) and (pi- proton to Delta gamma) to the
pion-nucleon scattering lengths of charged pions in the heavy baryon limit.
They give a large isospin violating contribution in the corresponding isoscalar
scattering length, but only a small violation in the isovector one. These terms
contribute 6.3(3)% to the 1s level shift of pionic hydrogen and give a chiral
constant F_pi^2f_1=-25.8(8) MeV.
