  We consider the minimal see-saw extension of the Standard Model with two
right-handed singlet fermions $N_{1,2}$ with mass at the GeV scale, augmented
by an effective dipole operator between the sterile states. We firstly review
current bounds on this effective interaction from fixed-target and collider
experiments as well as from astrophysical and cosmological observations. We
then highlight the prospects for testing the decay $N_2 \to N_1 \gamma$ induced
by the dipole at future facilities targeting long lived particles such as
ANUBIS, CODEX-b, FACET, FASER 2, MAPP and SHiP.
