The fundamental group of a log terminal T-variety

Abstract : We introduce an approach to study the fundamental group of a log terminal T-variety. As applications, we prove the simply connectedness of the spectrum of the Cox ring of a complex Fano variety, we compute the fundamental group of a rational log terminal T-variety of complexity one, and we study the local fundamental group of a log terminal T-singularity with a good torus action and trivial GIT decomposition.