Sous-groupe de Brauer invariant et obstruction de descente it\'er\'ee

For a quasi-projective smooth geometrically integral variety over a number field $k$, we prove that the iterated descent obstruction is equivalent to the descent obstruction. This generalizes a result of Skorobogatov, and this answers an open question of Poonen. The key idea is the notion of invariant Brauer subgroup and the notion of invariant 'etale Brauer-Manin obstruction for a $k$-variety equipped with an action of a connected linear algebraic group.