  Let $M^n$ be a n-dimensional compact manifold, with $n\geq3$. For any
conformal class C of riemannian metrics on M, we set $\mu_k^c(M,C)=\inf_{g\in
C}\mu_{[\frac n2],k}(M,g)\Vol(M,g)^{\frac2n}$, where $\mu_{p,k}(M,g)$ is the
k-th eigenvalue of the Hodge laplacian acting on coexact p-forms. We prove that
$0<\mu_k^c(M,C)\leq\mu_k^c(S^n,[g_{can}])\leq
k^{\frac2n}\mu_1^c(S^n,[g_{can}])$.
