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$|\Delta c|=|\Delta u|=1$ processes are unique probes of flavor physics inthe up-sector within and beyond the Standard Model (SM). SM tests with raresemileptonic charm meson decays are based on an approximate CP--symmetry, asuperior GIM--mechanism, angular distributions, lepton-universality and leptonflavor 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 theSM contribution. We identify signatures of supersymmetry, leptoquarks andanomaly-free $U(1)^\prime$-models with generation-dependent charges, for whichwe provide explicit examples. $Z^\prime$-effects in $c \to u \ell\ell^{(\prime)}$ transitions can be sizable if both left-handed andright-handed couplings to quarks are present.

The New Physics Reach of Null Tests with $D \to \pi \ell \ell$ and $D_s \to K \ell \ell $ Decays