Résumé
Let $G$ be a connected reductive subgroup of a complex connected reductive group $\hat{G}$. Fix maximal tori and Borel subgroups of $G$ and $\hat{G}$. Consider the cone $LR^\circ(\hat{G},G)$ generated by the pairs $(\nu,\hat{\nu})$ of strictly dominant characters such that $V_\nu$ is a submodule of $V_{\hat\nu}$. The main result of this article is a bijective parametrisation of the faces of $LR^\circ(\hat G,G)$. We also explain when such a face is contained in another one. In way, we obtain results about the faces of the Dolgachev-Hu's $G$-ample cone. We also apply our results to reprove known results about the moment polytopes.