  Heavy Neutral Leptons (HNLs) are a popular extension of the Standard Model to
explain the lightness of neutrino masses and the matter-antimatter asymmetry
through leptogenesis. Future direct searches, such as fixed target setups like
DUNE, and neutrinoless double beta decay are both expected to probe the regime
of active-sterile neutrino mixing in a standard Seesaw scenario of neutrino
mass generation for HNL masses around m_N <~ 1 GeV. Motivated by this, we
analyse the complementarity between future direct searches and neutrinoless
double beta decay to probe the nature of HNLs, i.e., whether they are Majorana
or quasi-Dirac states, and CP-violating phases in the sterile neutrino sector.
Following an analytic discussion of the complementarity, we implement a generic
fixed target experiment modelling DUNE. We perform a statistical study in how a
combined search for HNLs in direct searches and neutrinoless double beta decay,
using DUNE and LEGEND-1000 as representative examples, can probe the nature of
sterile neutrinos.
