  In the Standard Model minimally extended to include massive neutrinos, we
compute the leading CP-violating zero temperature contributions to the one-loop
effective action induced by integration of the leptons. Such contributions
start at operators of dimension six and they are P even for Dirac neutrinos and
P even or odd for Majorana neutrinos. Dimension four operators are allowed in
the mixed Dirac-Majorana case. It is verified by explicit calculation that CP
can be violated in two generation settings for Majorana neutrinos. Using
different neutrino scenarios we give upper bounds for the couplings of the
CP-violating operators. As a rule, we find that lepton-induced couplings are
suppressed as compared to quark-induced couplings, whenever the latter are
allowed, nevertheless, through virtual lepton-number violating mechanisms,
Majorana neutrinos induce new CP-violating operators not present in the quark
or Dirac-neutrino cases.
