Samples from Planck TT,TE,EE+lowE+lensing, colour coded by the value of the Hubble parameter H₀, for a model with minimal-mass active neutrinos and one additional sterile neutrino with mass parameterized by . The physical mass for thermally-produced sterile neutrinos, , is constant along the grey lines labelled by the mass in  eV; the equivalent result for sterile neutrinos produced via the Dodelson-Widrow mechanism (Dodelson & Widrow 1994) is shown by the adjacent thinner lines. The dark grey shaded region shows the part of parameter space excluded by our default prior , where the sterile neutrinos would start to behave like dark matter for CMB constraints.