  We study the lepton number violating muon-positron conversion in nuclei
mediated by the exchange of virtual light Majorana neutrinos. We found that a
previously overlooked imaginary part of this amplitude plays an important role.
The numerical calculation has been made for the experimentally interesting
muon-positron conversion in Ti48 using realistic renormalized proton-neutron
QRPA wave functions. We also discuss the very similar case of the neutrinoless
double beta decay of Ca48 The ratio of muon-positron conversion over the total
muon absorption has been computed taking into account the current constraints
from neutrino oscillation phenomenology. We compare our results with the
experimental limits as well as with previous theoretical predictions. We have
found that the Majorana neutrino mode of muon-positron conversion in Ti48 is
too small to be measurable in the foreseeable future.
