  A metric is formal if all products of harmonic forms are again harmonic. The
existence of a formal metric implies Sullivan formality of the manifold, and
hence formal metrics can exist only in presence of a very restricted topology.
We show that a warped product metric is formal if and only if the warping
function is constant and derive further topological obstructions to the
existence of formal metrics. In particular, we determine necessary and
sufficicient conditions for a Vaisman metric to be formal.
