We provide in this paper a counterexample to the Benson-Ratcliff conjecture about a cohomology class invariant on coadjoint orbits on nilpotent Lie groups. We prove that this invariant never vanishes on generic coadjoint orbits for some restrictive classes. As such, it does separate up to invariant factor, unitary representations associated to generic orbits in some cases.
"On the Benson-Ratcliff invariant of coadjoint orbits on nilpotent Lie groups." Osaka J. Math. 44 (2) 399 - 414, June 2007.