  Let $G$ be a connected reductive group over a number field $F$, and let $S$
be a set (finite or infinite) of places of $F$. We give a necessary and
sufficient condition for the surjectivity of the localization map from
$H^1(F,G)$ to the "direct sum" of the sets $H^1(F_v,G)$ where $v$ runs over
$S$. In the appendices, we give a new construction of the abelian Galois
cohomology of a reductive group over a field of arbitrary characteristic.
