  We consider a scenario of Low Scale Seesaw where the masses of its heavy
Majorana neutrinos are arranged in a pattern of three pairs of quasi degenerate
ones, in the range of O(1-6 GeV). Since they can violate lepton number by two
units, they contribute to rare decays of Ds and Bc mesons, providing, besides,
conditions for maximal CP violation. We found that new phenomenology is
possible depending on how many of on-shell pairs mediate these decays. In
particular, we present new constraints on muon-heavy neutrinos mixing
parameters.
