  The extension of the Standard Model by right-handed neutrinos can not only
explain the active neutrino masses via the seesaw mechanism, it is also able
solve a number of long standing problems in cosmology. Especially, masses below
the TeV scale are of particular interest as they can lead to a plethora of
signatures in experimental searches. We present the first full frequentist
analysis of the extension of the Standard Model by three right-handed
neutrinos, with masses between 60 MeV and 500 GeV, using the Global and Modular
BSM (beyond the Standard Model) Inference Tool GAMBIT. Our analysis is based on
the Casas-Ibarra parametrisation and includes a large range of experimental
constraints: active neutrino mixing, indirect constraints from, e.g.,
electroweak precision observables and lepton universality, and numerous direct
searches for right-handed neutrinos. To study their overall effect, we derive
combined profile likelihood results for the phenomenologically most relevant
parameter projections. Furthermore, we discuss the role of (marginally)
statistically preferred regions in the parameter space. Finally, we explore the
flavour mixing pattern of the three right-handed neutrinos for different values
of the lightest neutrino mass. Our results comprise the most comprehensive
assessment of the model with three right-handed neutrinos model below the TeV
scale so far, and provide a robust ground for exploring the impact of future
constraints or detections.
