  A poorly constrained parameter in the Standard Model of Cosmology is the
lepton asymmetry l = \sum_f l_f=\sum_f(n_f+n_{\nu_f})/s. Each flavour asymmetry
l_f with f=e, \mu, {\tau} is the sum of the net particle density of the charged
leptons n_f and their corresponding neutrinos, normalized with the entropy
density s. Constraints on l_f \leq O(0.1) from BBN and CMB allow for lepton
flavour asymmetries orders of magnitudes larger then the baryon asymmetry b ~
10^{-10}. In this article we show how such large lepton (flavour) asymmetries
influence the early universe, in particular the freeze out of WIMPs and the
cosmic QCD transition.
