  We discuss a little-studied class of weak decay modes sensitive to only one
quark topology at leading order in G_F: M --> m gamma, where M,m are mesons
with completely distinct flavor quantum numbers. Specifically, they proceed via
the annihilation of the valence quarks through a W and the emission of a single
hard photon, and thus provide a clear separation between CKM and strong
interaction physics. We survey relevant calculations performed to date, discuss
experimental discovery potential, and indicate interesting future directions.
