$N_{\rm eff}$ in low-scale seesaw models versus the lightest neutrino
  mass
  We evaluate the contribution to $N_{\rm eff}$ of the extra sterile states in
low-scale Type I seesaw models (with three extra sterile states). We explore
the full parameter space and find that at least two of the heavy states always
reach thermalisation in the Early Universe, while the third one might not
thermalise provided the lightest neutrino mass is below ${\mathcal
O}(10^{-3}$eV). Constraints from cosmology therefore severely restrict the
spectra of heavy states in the range 1eV- 100 MeV. The implications for
neutrinoless double beta decay are also discussed.
