We construct a birational
invariant for certain algebraic group actions. We use this invariant to classify linear
representations of finite abelian groups up to birational equivalence, thus
answering, in a special case, a question of E.B. Vinberg and giving a family of
counterexamples to a related conjecture of P.I. Katsylo. We also give a new
proof of a theorem of M. Lorenz on birational equivalence of quantum tori
(in a slightly expanded form) by applying our invariant in the setting of
PGLn-varieties.