  We point out that the rare decay mode $W^{\pm} \rightarrow \gamma +
D^{\pm}_{s} $ could be spectacularly enhanced, with a branching ratio around
$10^{-6} $ which is three order of magnitude larger than previous predictions.
Its observation will determine the W boson mass with great accuracy, providing
additional high precision tests of the standard model, as well as reveal
eventual deviation from the trilinear non-abelian gauge coupling.
