Effective models of group schemes

29 pages. Supersedes previous preprint "Effective model of a finite group action"Let $R$ be a discrete valuation ring with fraction field $K$ and $X$ a flat $R$-scheme. Given a faithful action of a $K$-group scheme $G_K$ over the generic fibre $X_K$, we study models $G$ of $G_K$ acting on $X$. In various situations, we prove that if such a model $G$ exists, then there exists another model $G'$ that acts faithfully on $X$. This model is the schematic closure of $G$ inside the fppf sheaf $Aut_R(X)$~; the major difficulty is to prove that it is representable by a scheme. For example, this holds if $X$ is locally of finite type, separated, flat and pure and $G$ is finite flat. Pure schemes (a notion recalled in the text) have many nice properties : in particular, we prove that they are the amalgamated sum of their generic fibre and the family of their finite flat closed subschemes. We also provide versions of our results in the setting of formal schemes