An interesting family of conformally invariant one-forms in even dimensions

We construct a natural conformally invariant one-form of weight −2k on any 2k-dimensional pseudo-Riemannian manifold which is closely related to the Pfaffian of the Weyl tensor. On oriented manifolds, we also construct natural conformally invariant one-forms of weight −4k on any 4k-dimensional pseudo-Riemannian manifold which are closely related to top degree Pontrjagin forms. The weight of these forms implies that they define functionals on the space of conformal Killing fields. On Riemannian manifolds, we show that this functional is trivial for the former form but not for the latter forms. As a consequence, we obtain global obstructions to the existence of an Einstein metric in a given conformal class.