  We investigate Cabibbo suppressed $D^{0}$, $D^{+}$ and $D^{+}_{s}$ radiative
weak decays in order to find the best mode to test $c\to u \gamma$ decay.
Combining heavy quark effective theory and the chiral Lagrangian approach we
determine the decay widths. We calculate $\Gamma(D^{0}\to
\rho^{0}/\omega\gamma)/ \Gamma(D^{0}\to {\bar K}^{*0} \gamma)$ previously
proposed to search for possible New Physics. However, we notice that there are
large, unknown, corrections within the Standard Model. We propose a better
alternative, the ratio $\Gamma(D_{s}^{+}\to K^{*+} \gamma)/ \Gamma(D_{s}^{+}\to
\rho^{+} \gamma)$, and show that it is less sensitive to the Standard Model.
