  We consider superconductivity in the presence of impurities in a two-band
model suited for the description of iron-based superconductors. We analyze the
effect of interband scattering processes on superconductivity, allowing for
orbital, i.e., nonspin-magnetic but time-reversal symmetry-breaking impurities.
Pair breaking in such systems is described by a nontrivial phase in an
interband-scattering matrix element. We find that the transition temperature of
conventional superconductors can be suppressed due to interband scattering,
whereas unconventional superconductors may be unaffected. We also discuss the
stability of density wave phases in the presence of impurities. As an example,
we consider impurities associated with imaginary charge density waves that are
of interest for iron-based superconductors.
