  Let $(M,g)$ be an $n$-dimensional $(n\geq 3)$ compact Riemannian manifold
with Ric$_{(M,g)}\geq (n-1)g$. If $(M,g)$ supports an AB-type critical Sobolev
inequality with Sobolev constants close to the optimal ones corresponding to
the standard unit sphere $(\mathbb S^n,g_0)$, we prove that $(M,g)$ is
topologically close to $(\mathbb S^n,g_0)$. Moreover, the Sobolev constants on
$(M,g)$ are precisely the optimal constants on the sphere $(\mathbb S^n,g_0)$
if and only if $(M,g)$ is isometric to $(\mathbb S^n,g_0)$; in particular, the
latter result answers a question of V.H. Nguyen.
