a light neutrino.

A few more comments on Tab. \[Tab:TopoI\] might be in order. There are a total of six possibilities in which the intermediate fermion transforms $\psi(0,{\bf 1})$ under the gauge symmetries $(U(1)_{\rm em}, SU(3)_{c})$. Only they can lead to long-range contributions, all the other models in the list are necessarily of the short-range type. Among those six, only the cases marked (a) or (b) in the column “Long Range?” can lead to interesting constraints, since the remaining three cases marked as (c) are suppressed with $\epsilon^2$. Note, however, that all of these cases can also be of short-range type. There are 12 (times two) cases listed, which require that the internal fermion has a fractional electric charge. As far as we know, none of them have been discussed in the literature before. All of these new “models” not only require fractionally charged fermions, but also exotic bosons. The latter can be doubly-charged bileptons [@Cuypers:1996ia], diquarks, or leptoquarks [@Buchmuller:1986zs].

The [$0\nu\beta\beta$ ]{}decay process violates lepton number $L$. We can easily identify the different possibilities for LNV from Tab. \[Tab:TopoI\] (cf., Fig. \[Fig:0nbbTopologies\], left panel). If the internal fermion is neutral, it can have a Majorana mass and a mass 