Abstract. This article shows how to approximate a stable action of a finitely presented group on an R-tree by a simplicial one while keeping control over arc stabilizers. For instance, every small action of a hyperbolic group on an R-tree can be approximated by a small action of the same group on a simplicial tree. The techniques we use highly rely on Rips's study of stable actions on R-trees and on the dynamical study of exotic components by D. Gaboriau.
Mathematics Subject Classification (1991). 20E08, 20E06, 20F32, 05C05.Keywords. Stable action, R-trees, Rips theorem, approximation, splitting, Bass-Serre.M. Bestvina and M. Feighn introduced the notion of stable action of a group on an R-tree ([BF2]) by slightly weakening some conditions by E. Rips or by H. Gillet and P. Shalen. Roughly speaking, stable actions are such that the stabilizer of an arc must stabilize when this arc gets smaller and smaller (see section 1 for a formal definition). This condition is true in usual cases: any action with trivial arc stabilizer, the actions coming from iteration of automorphisms of free groups or from degeneracy of hyperbolic structures, and every small action of a hyperbolic group is stable (see section 1).The main theorem about stable actions is Rips's theorem (see [BF2]): if a finitely presented group Γ has a non trivial stable action on an R-tree, then Γ splits over a group C which is an extension of Z k by a subgroup of Γ fixing an arc in T .On the other hand, M. Cohen and M. Lustig have introduced very small actions on R-trees and have shown that the set of free actions of the free group F n on simplicial R-trees is dense in the space of very small actions of F n on simplicial Rtrees ([CL]). Then, M. Bestvina and M. Feighn showed that every very small action of F n on an R-tree can be approximated by a very small action on a simplicial R-tree ([BF3]). This showed that the closure of M. Culler and K. Vogtmann's outer space is the projectivised set of very small actions of F n on R-trees.Our theorem is both a refinement of E. Rips' splitting theorem, and a generalisation of M. Bestvina and M. Feighn's approximation theorem: Theorem 1. Let Γ be a finitely presented group. Every minimal stable action of