Mixing lepton doublets of the Standard Model can lead to lepton flavour asymmetries in the Early Universe .
We present a diagrammatic representation of this recently identified source of CP violation and elaborate in detail on the correlations between the lepton flavours at different temperatures .
For a model where two sterile right-handed neutrinos generate the light neutrino masses through the see-saw mechanism , the lower bound on reheat temperatures in accordance with the observed baryon asymmetry turns out to be \raisebox { -3.0 pt } { $ \stackrel { \displaystyle > } { \sim } $ } 1.2 \times 10 ^ { 9 } { GeV } .
With three right-handed neutrinos , substantially smaller values are viable .
This requires however a tuning of the Yukawa couplings , such that there are cancellations between the individual contributions to the masses of the light neutrinos .