  $|\Delta c|=|\Delta u|=1$ processes are unique probes of flavor physics in
the up-sector within and beyond the Standard Model (SM). SM tests with rare
semileptonic charm meson decays are based on an approximate CP--symmetry, a
superior GIM--mechanism, angular distributions, lepton-universality and lepton
flavor conservation. We analyze the complete set of null test observables in $D
\to \pi \ell \ell^{(\prime)}$ and $D_s \to K \ell \ell^{(\prime)}$ decays,
$\ell^{(\prime)}=e, \mu$, and find large room for new physics safely above the
SM contribution. We identify signatures of supersymmetry, leptoquarks and
anomaly-free $U(1)^\prime$-models with generation-dependent charges, for which
we provide explicit examples. $Z^\prime$-effects in $c \to u \ell
\ell^{(\prime)}$ transitions can be sizable if both left-handed and
right-handed couplings to quarks are present.
