Let $\Gamma$ be a Coxeter graph, let $W$ be its associated Coxeter group, and let $G$ be a group of symmetries of $\Gamma$. Recall that, by a theorem of Hée and M\"uhlherr, $W^G$ is a Coxeter group associated to some Coxeter graph $\hat \Gamma$. We denote by $\Phi^+$ the set of positive roots of $\Gamma$ and by $\hat \Phi^+$ the set of positive roots of $\hat \Gamma$. Let $E$ be a vector space over a field $\K$ having a basis in one-to-one correspondence with $\Phi^+$. The action of $G$ on $\Gamma$ induces an action of $G$ on $\Phi^+$, and therefore on $E$. We show that $E^G$ contains a linearly independent family of vectors naturally in one-to-one correspondence with $\hat \Phi^+$ and we determine exactly when this family is a basis of $E^G$. This question is motivated by the construction of Krammer's style linear representations for non simply laced Artin groups.