Résume | Given a $p$-adic representation of the Galois group of a local field, we show that its Galois cohomology can be computed using the associated étale $(\varphi,\Gamma)$-module over the Robba ring; this is a variant of a result of Herr. We then establish analogues, for not necessarily étale $(\varphi,\Gamma)$-modules over the Robba ring, of the Euler-Poincaré characteristic formula and Tate local duality for $p$-adic representations. |