QUARK OFF-SHELL EFFECTS IN FLAVOUR-CHANGING DECAYS

We discuss some flavour-changing effective Lagrangians, obtained from scales above 1 GeV, which vanish on the quark mass shell. Although the effects of such effective Lagrangians are zero in the limit of vanishing bound-state interactions, we have shown that they have a significant impact on the processes $K\rightarrow\gamma\gamma$ and $B_s \rightarrow\gamma\gamma$.