  We discuss bounds to strong gravity arising from annihilation of electron and
positrons and the corresponding enhancement of the cross section for the
production of massive particles. Two complementary examples are discussed, the
case of the $e^+e^- \to W^+ W^-$ processes, with maximum mass and short
lifetime for the particle pairs in the final state, and the case of electron
positron annihilation into heavier charged lepton pairs, $e^+ e^- \to \mu^+
\mu^-$ and $e^+ e^- \to \tau^+ \tau^-$. These bounds may be improved by the
next generation of electron-positron colliders, and are complementary to bounds
arising from direct graviton searches.
