Left: 2D joint posterior distribution between N_eff and ∑ m_ν (the summed mass of the three active neutrinos) in models with extra massless neutrino-like species. Right: samples in the N_eff- plane, colour-coded by Ω_ch², in models with one massive sterile neutrino family, with effective mass , and the three active neutrinos as in the base ΛCDM model. The physical mass of the sterile neutrino in the thermal scenario, , is constant along the grey dashed lines, with the indicated mass in eV. The physical mass in the Dodelson-Widrow scenario, , is constant along the dotted lines (with the value indicated on the adjacent dashed lines).Note the pile up of points at low values of N_eff, caused because the sterile neutrino component behaves like cold dark matter there, introducing a strong degeneracy between the two components, as described in the text.