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Simultaneous constraints on the number and mass of relativistic species

Recent indications from both particle physics and cosmology suggest the existence of more than three neutrino species. In cosmological analyses the effects of neutrino mass and number of species can in principle be disentangled for fixed cosmological parameters. However, since we do not have perfect measurements of the standard Lambda Cold Dark Matter model parameters some correlation remains between the neutrino mass and number of species, and both parameters should be included in the analysis. Combining the newest observations of several cosmological probes (cosmic microwave background, large scale structure, expansion rate) we obtain N_eff=3.58(+0.15/-0.16 at 68% CL) (+0.55/-0.53 at 95% CL) and a sum of neutrino masses of less than 0.60 eV (95 CL), which are currently the strongest constraints on N_eff and M_nu from an analysis including both parameters. The preference for N_eff >3 is now at a 2sigma level.