  Recent observations of the cosmic microwave background (CMB) at smallest
angular scales and updated abundances of primordial elements, indicate an
increase of the energy density and the helium-4 abundance with respect to
standard big bang nucleosynthesis with three neutrino flavour. This calls for a
reanalysis of the observational bounds on neutrino chemical potentials, which
encode the number asymmetry between cosmic neutrinos and anti-neutrinos and
thus measures the lepton asymmetry of the Universe. We compare recent data with
a big bang nucleosynthesis code, assuming neutrino flavour equilibration via
neutrino oscillations before the onset of big bang nucleosynthesis. We find a
slight preference for negative neutrino chemical potentials, which would imply
an excess of anti-neutrinos and thus a negative lepton number of the Universe.
This lepton asymmetry could exceed the baryon asymmetry by orders of magnitude.
