Do WMAP data constraint the lepton asymmetry of the Universe to be zero?

It is shown that extended flat ΛCDM models with massive neutrinos, a sizeable lepton asymmetry and an additional contribution to the radiation content of the Universe, are not excluded by the Wilkinson Microwave Anisotropy Probe (WMAP) first year data. We assume a flat cosmological model with three thermally distributed neutrino species having all the same mass and chemical potential, plus an additional amount of effectively massless exotic particle species X. After maximizing over seven other cosmological parameters, we derive from WMAP first year data the following constraints for the lepton asymmetry L of the Universe (95% CL): 0 < |L| < 0.9, so that WMAP data alone cannot firmly rule out scenarios with a large lepton number; moreover, a small preference for this kind of scenarios is actually found. We also find for the neutrino mass mν < 1.2 eV and for the effective number of relativistic particle species -0.45 < ΔNeff < 2.10, both at 95% CL. The limit on ΔNeff is more restrictive man others found in the literature, but we argue that this is due to our choice of priors.