  The leading non-Coulombic electromagnetic contributions to pi pi s-wave
scattering lengths are computed in Chiral Perturbation Theory. It is shown that
these corrections are zeroth order in the chiral expansion and associated with
electromagnetic contact terms in the effective Lagrangian, i.e. that they do
not involve explicit photon fields in the low-energy effective theory. It is
pointed out that, if one followed the standard meson-exchange-model ansatz for
removing electromagnetic effects, i.e. of subtracting contributions associated
with explicit photon exchange and radiative corrections, as determined by the
photon coupling vertices of the effective hadronic theory, one would completely
miss these contributions and arrive at the erroneous conclusion that the strong
interactions exhibited very large isospin breaking in pi pi scattering.
Implications for electromagnetic "subtraction" procedures in other hadronic
systems and the utility of the effective Lagrangian method for avoiding such
errors are obvious.
