The Karmen anomaly can be interpreted as being due to a heavy neutrino of mass around 137 MeV produced in πe2 decays. This interpretation is consistent with the present limits on the couplings of such an object.
@article{arxiv.hep-ph/0003194,
title = {A consistent interpretation of the Karmen anomaly},
author = {J. -M. Levy and F. Vannucci},
journal= {arXiv preprint arXiv:hep-ph/0003194},
year = {2007}
}