International audienceWe classify all smooth projective horospherical varieties with Picard number 1. We prove that the automorphism group of any such variety X acts with at most two orbits and that this group still acts with only two orbits on X blown up at the closed orbit. We characterize all smooth projective two-orbit varieties with Picard number 1 that satisfy this latter property

Résumé. -Une variété horosphérique est une variété algébrique normale dans laquelle un groupe algébrique réductif opère avec une orbite ouverte fibrée en tores sur une variété de drapeaux. En particulier, les variétés toriques et les variétés de drapeaux sont horosphériques. Dans cet article, on classifie les variétés horosphériques de Fano en termes de certains polytopes rationnels qui généralisent les polytopes ré-flexifs considérés par V. Batyrev. Puis on obtient une majoration du degré des variétés horosphériques lisses de Fano, analogue à celle donnée par O. Debarre dans le cas torique. On étend un résultat récent de C. Casagrande: les variétés horosphériques Q-factorielles de Fano ont leur nombre de Picard majoré par deux fois la dimension.Abstract (Fano horospherical varieties). -A horospherical variety is a normal algebraic variety where a reductive algebraic group acts with an open orbit which is a torus bundle over a flag variety. For example, toric varieties and flag varieties are horospherical. In this paper, we classify Fano horospherical varieties in terms of certain rational polytopes that generalize the reflexive polytopes considered by V. Batyrev. Then, we obtain an upper bound on the degree of smooth Fano horospherical varieties, analogus to that given by O. Debarre in the toric case. We extend a recent result of C. Casagrande: the Picard number of any Fano Q-factorial horospherical variety is bounded by twice the dimension.

Let G be a complex connected reductive group. The Parthasarathy-Ranga RaoVaradarajan (PRV) conjecture, which was proved independently by S. Kumar and O. Mathieu in 1989, gives explicit irreducible submodules of the tensor product of two irreducible G-modules. This paper has three aims. First, we simplify the proof of the PRV conjecture, then we generalize it to other branching problems. Finally, we find other irreducible components of the tensor product of two irreducible G-modules that appear for 'the same reason' as the PRV ones.

For a G-variety X with an open orbit, we define its boundary ∂X as the complement of the open orbit. The action sheaf S X is the subsheaf of the tangent sheaf made of vector fields tangent to ∂X. We prove, for a large family of smooth spherical varieties, the vanishing of the cohomology groups H i (X, S X ) for i > 0, extending results of F. Bien and M. Brion [BB96]. We apply these results to study the local rigidity of the smooth projective varieties with Picard number one classified in [Pa08b].

We use the Grossberg-Karshon's degeneration of Bott-Samelson varieties to toric varieties and the description of cohomology of line bundles on toric varieties to deduce vanishing results for the cohomology of lines bundles on Bott-Samelson varieties.