  The nuclear physics dependence of the exotic $(\mu^-,e^-)$ conversion
branching ratio $R_{\mu e^-}$ for the experimentally most interesting nuclei
$^{208}Pb$ and $^{48}Ti$, is investigated in various nuclear models. The
results thus obtained are combined with the new experimental limits extracted
at PSI for these nuclei to put bounds on the elementary particle parameters
entering $R_{\mu e^-}$ such as intermediate neutrino masses and mixing angles
as well as relevant parameters of intermediate supersymmetric particles (masses
and mixing of s-fermions and neutralinos).
