  The Landau-Yang theorem, forbidding transition amplitudes between a massive
spin-1 particle and two photons, is widely assumed to apply to other massless
spin-1 particles. We show that this is not true in Standard-Model QCD, so that
for instance antisymmetric colour-octet spin-1 quarkonia can be formed by two
on-shell gluons in second-order perturbative QCD.
