  We estimate the electromagnetic effect of the spectator charge on the
momentum spectra of charged pions produced in peripheral Pb+Pb collisions at
SPS energies. We find a large effect which results in strongly varying
structures in the $x_F$ dependence of the $\pi^+/\pi^-$ ratio, especially at
low transverse momenta where a deep valley in the above ratio is predicted at
$x_F \sim$ 0.15 -- 0.20. The effect depends on initial conditions. Thus, it
provides new information on the space and time evolution of the
non-perturbative pion creation process.
