  We study the impact of TeV-scale sterile neutrinos on electro-weak precision
observables and lepton number and flavour violating decays in the framework of
a type-I see-saw extension of the Standard Model. At tree level sterile
neutrinos manifest themselves via non-unitarity of the PMNS matrix and at
one-loop level they modify the oblique radiative corrections. We derive
explicit formulae for the S,T,U parameters in terms of the neutrino masses and
mixings and perform a numerical fit to the electro-weak observables. We find
regions of parameter space with a sizable active-sterile mixing which provide a
better over-all fit compared to the case where the mixing is negligible.
Specifically we find improvements of the invisible Z-decay width, the
charged-to-neutral-current ratio for neutrino scattering experiments and of the
deviation of the W boson mass from the theoretical expectation.
