Abstract : Let $(W,S)$ be a Coxeter system, let $G$ be a group of symmetries of $(W,S)$ and let $f : W \to \GL (V)$ be the linear representation associated with a root basis $(V, \langle .,. \rangle, \Pi)$.
We assume that $G \subset \GL (V)$, and that $G$ leaves invariant $\Pi$ and $\langle .,. \rangle$.
We show that $W^G$ is a Coxeter group, we construct a subset $\tilde \Pi \subset V^G$ so that $(V^G, \langle .,. \rangle, \tilde \Pi)$ is a root basis of $W^G$, and we show that the induced representation $f^G : W^G \to \GL( V^G)$ is the linear representation associated with $(V^G, \langle .,. \rangle, \tilde \Pi)$.
In particular, the latter is faithful.
The fact that $W^G$ is a Coxeter group is already known and is due to M\"uhlherr and H\'ee, but also follows directly from the proof of the other results.