Bound-state effects in mu(+)e(-)->gamma gamma and (B)over-bar(s)(0)->gammagamma decays

We demonstrate that in the double-radiative decays of heavy-light QED and Q CD atoms, mu (+)e(-) --> gamma gamma and (B) over bar (0)(s) --> gamma gamm a, there is a contribution coming from operators that vanish on the free-qu ark mass shell. This off-shell effect is suppressed with respect to the eff ect, of the well-known flavour-changing magnetic-moment operator by the bou nd-state binding factor. Accordingly, the negligible off-shellness of the w eakly bound QED atoms becomes important for strongly bound QCD atoms. We pr esent this effect in two different model approaches to QCD, one of them ena bling us to keep close contact to the related effect in QED.