  The effective Lagrangian for the seesaw model is derived including effects
due to CP violation. Besides the usual dimension-5 operator responsible for
light neutrino masses, a dimension-6 operator is obtained. For three or less
heavy neutrino generations, the inclusion of both operators is necessary and
sufficient for all independent physical parameters of the high-energy seesaw
Lagrangian to appear in the low-energy effective theory, including the CP-odd
phases relevant for leptogenesis. The dimension-6 operator implies exotic
low-energy couplings for light neutrinos, providing a link between the
high-energy physics and low-energy observables.
