  Majorons are the Goldstone bosons associated to lepton number and thus
closely connected to Majorana neutrino masses. Couplings to charged fermions
arise at one-loop level, including lepton-flavor-violating ones that lead to
decays $\ell\to \ell' J$, whereas a coupling to photons is generated at two
loops. The typically small couplings make massive majorons a prime candidate
for long-lived dark matter. Its signature decay into two mono-energetic
neutrinos is potentially detectable for majoron masses above MeV.
