  The experimental observation of lepton-number violating processes would
unambiguously indicate the Majorana nature of neutrinos. Various $\Delta L$ = 2
processes for pseudoscalar meson $M_1$ decays to another pseudoscalar meson
$M_2$ and two charged leptons $\ell_1$, $\ell_2$ ($M_1^+\to
\ell_1^+\ell_2^+M_2^-$) have been studied extensively. Extending the existing
literature on the studies of these kinds of processes, we consider the rare
decays of heavy mesons to a vector meson or a pseudoscalar meson. These
processes have not been searched for experimentally, while they may have
sizable decay rates. We calculate their branching fractions and propose to
search for these decay modes in the current and forthcoming experiments, in
particular at the LHCb.
