  Let $R$ be a discrete valuation ring with fraction field $K$. Let $X$ be a
flat $R$-scheme of finite type and $G$ a finite flat group scheme acting on $X$
so that $G\_K$ is faithful on the generic fibre $X\_K$. We prove that there is
an effective model of $G$ i.e. a finite flat group scheme dominated by $G$,
isomorphic to it on the generic fibre, and extending the action of $G\_K$ on
$X\_K$ to an action on all of $X$ that is faithful also on the special fibre.
It is unique with these properties. We give examples and applications to
degenerations of coverings of curves.
