Unitarity triangles and geometrical description of CP violation with   Majorana neutrinos
  We generalize the geometrical description of CP violation in the Standard Model in terms of a unitarity triangle. For three left-handed Majorana neutrinos CP violation in the lepton sector is determined by three unitarity triangles. With three additional right-handed neutrinos 15 quadrangles are required to characterize CP violation. We show the relation of the unitarity polygons with physical observables. 
J. A. Aguilar-Saavedra, G. C. Branco