  We study experimental and cosmological constraints on the extension of the
Standard Model by three right handed neutrinos with masses between those of the
pion and W boson. We combine for the first time direct, indirect and
cosmological constraints in this mass range. This includes experimental
constraints from neutrino oscillation data, neutrinoless double $\beta$ decay,
electroweak precision data, lepton universality, searches for rare lepton
decays, tests of CKM unitarity and past direct searches at colliders or fixed
target experiments. On the cosmological side, big bang nucleosynthesis has the
most pronounced impact. Our results can be used to evaluate the discovery
potential of searches for heavy neutrinos at LHCb, BELLE II, SHiP, ATLAS, CMS
or a future lepton collider.
