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Coxeter groups and automorphisms

Let $(W,S)$ be a Coxeter system and $Γ$ be a group of automorphisms of $W$ such that $γ(S)=S$ for all $γ\in Γ$. Then it is known that the group of fixed points $W^Γ$ is again a Coxeter group with a canonically defined set of generators. The usual proofs of this fact rely on the reflection representation of $W$. Here, we give a proof which only uses the combinatorics of reduced expressions in $W$. As a by-product, this shows that the length function on $W$ restricts to a weight function on $W^Γ$.