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Low-scale seesaw models versus $N_{\rm eff}$

We consider the contribution of the extra sterile states in generic low-scale seesaw models to extra radiation, parametrized by $N_{\rm eff}$. We find that the value of $N_{\rm eff}$ is roughly independent of the seesaw scale within a wide range. We explore the full parameter space in the case of two extra sterile states and find that these models are strongly constrained by cosmological data for any value of the seesaw scale below ${\mathcal O}(100$MeV).