  We consider the problem of counting the number of rational points of bounded
height in the zero-loci of Brauer group elements on semi-simple algebraic
groups over number fields. We obtain asymptotic formulae for the counting
problem for wonderful compactifications using the spectral theory of
automorphic forms. Applications include asymptotic formulae for the number of
matrices over Q whose determinant is a sum of two squares. These results
provide a positive answer to some cases of a question of Serre concerning such
counting problems.
