i will present recent joint work with arnaud brothier on actions of locally compact groups on von neumann algebras. we study algebraic properties of the associated crossed-product algebras, and prove among other things a correspondance result between certain subalgebras of this crossed-product and closed subgroups of the acting group. this generalizes results of izumi-longo-popa. i will explain our (very different) approach, and give related questions and conjectures.