An attractive way to generate neutrino masses as required to account for current neutrino oscillation data involves the spontaneous breaking of lepton number. The resulting majoron may pick up a mass due to gravity. If its mass lies in the kilovolt scale, the majoron can play the role of late-decaying dark matter (LDDM), decaying mainly to neutrinos. In general the majoron has also a sub-dominant decay to two photons leading to a mono-energetic emission line which can be used as a test of the LDDM scenario. We compare expected photon emission rates with observations in order to obtain model-independent restrictions on the relevant parameters. We also illustrate the resulting sensitivities within an explicit seesaw realization, where the majoron couples to photons due to the presence of a Higgs triplet. © 2008 IOP Publishing Ltd. and SISSA.