  We estimate the electromagnetic effect of the spectator charge on the
momentum spectra of $\pi^+$ and $\pi^-$ produced in peripheral Pb+Pb collisions
at SPS energies. We find that the effect is large and 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.
  It appears that the effect depends on initial conditions. Thus, it provides
new information on the space and time evolution of the non-perturbative pion
creation process.
