  Leptonic decays of $B$-mesons are theoretically very clean probes for testing
the Standard Model (SM) and possible physics beyond it. Amongst the various
leptonic decays of the $B$-meson, the pure dileptonic decay $B \to \ell^+
\ell^-$ is very important as this mode is helicity suppressed in the SM but can
be substantially enhanced in some of the models beyond the SM, such as
supersymmetric (SUSY) theories and the two Higgs doublet model (2HDM). Although
the purely dileptonic decay mode is helicity suppressed in the SM its
associated mode $B \to \ell^+ \ell^- \gamma$ does not have the same
suppression, due to the presence of the $\gamma$ in the final state. In this
paper we will also analyse the effects of enhanced $Z$-penguins on these two
decay modes.
