  We analyze CP violation in resonant transitions involving scalar as well as
fermionic intermediate states, using a gauge-invariant resummation approach
implemented by the pinch technique. We derive the necessary conditions for
resonantly enhanced CP violation induced by particle mixing, by paying special
attention to CPT invariance, and apply the results of our analysis to two
representative new-physics scenarios: (i) the indirect mixing of a CP-even
Higgs scalar with a CP-odd Higgs particle in two-Higgs doublet models and (ii)
the CP-asymmetric mixing between the top quark and a new heavy up-type quark,
$t'$, in mirror fermion models. Furthermore, we explicitly demonstrate the
equivalence of our scattering-amplitude formalism with that based on the
effective Hamiltonian and discuss the possibility of maximal CP violation in
the limiting case of two degenerate particles.
