Let $(W,S)$ be a finite irreducible CoxeterSystem of rank $n$ and $E$ be a real reflection representation of $W$. Let $x\in E$ and suppose that the isotropy group of $x$ is generated by one element in $S$. Now which are the subgroups of rank $n1$ that do not stabilize $x$?
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You have to consider the Coxeter Graph: The subgroups you are looking for are those that you get by removing one edge... 

