  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 $\gsim 1.2\times
10^9\,{\rm GeV}$. 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.
