We analyze CP violation in resonant transitions involving scalar as we
ll as fermionic intermediate states, using a gauge-invariant resummati
on approach implemented by the pinch technique. We derive the necessar
y conditions for resonantly enhanced CP violation induced by particle
mixing, by paying special attention to CPT invariance, and apply the r
esults of our analysis to two representative new-physics scenarios: (i
) the indirect mixing of a CP-even Higgs scalar with a CP-odd Higgs pa
rticle in two-Higgs doublet models and (ii) the CP-asymmetric mixing b
etween the top-quark and a new heavy up-type quark, t', in mirror ferm
ion models. Furthermore, we explicitly demonstrate the equivalence of
our scattering-amplitude formalism with that based on the effective Ha
miltonian and discuss the possibility of maximal CP violation in the l
imiting case of two degenerate particles. (C) 1997 Elsevier Science B.
V. PACS: 11.30.Er; 13.10.+q; 14.80.Bn; 14.80.Cp.