Calcul d'une valeur d'un facteur epsilon par une formule int\'egrale

Let d and m be two natural numbers of distinct parities. Let $\pi$ be an admissible irreducible tempered representation of GL(d,F), where F is a p-adic field. We assume that $\pi$ is self-dual. Then we can extend $\pi$ as a representation $\tilde{\pi}$ of a non-connected group $GL(d,F)\rtimes \{1,\theta\}$. Let $\rho$ be a representation of GL(m,F). We assume that it has similar properties as $\pi$. Jacquet, Piatetski-Shapiro and Shalika have defined the factor $\epsilon(s,\pi\times\rho,\psi)$. We prove that we can compute $\epsilon(1/2,\pi\times\rho,\psi)$ by an integral formula where occur the characters of $\tilde{\pi}$ and $\tilde{\rho}$. It's similar to the formula which, for special orthogonal groups, computes the multiplicities appearing in the local Gross-Prasad conjecture.