Equivariant automorphism group and real forms of complexity-one varieties

Abstract: Let G be a connected reductive algebraic group over a perfect field. We study the representability of the equivariant automorphism group of G-varieties. For a broad class of complexity-one G-varieties, we show that this group is representable by a group scheme locally of finite type when the base field has characteristic zero. We also establish representability, by a linear group, in the case of almost homogeneous G-varieties of arbitrary complexity. Using an exact sequence description of the equivariant automorphism group, we derive a criterion to determine whether a complexity-one variety admits only finitely many real forms. In contrast, we present a very simple example of a complexity one surface having infinitely many pairwise non-isomorphic real forms.