  The study of mesons and baryons which contain at least one charm quark is
referred to as open charm physics. It offers the possibility to study up-type
quark transitions. Since the $c$ quark can not be treated in any mass limit,
theoretical predictions are difficult and experimental input is crucial. BESIII
collected large data samples of $e^{+}e^{-}$ collisions at several charm
thresholds. The at-threshold decay topology offers special opportunities to
study open charm decays. We present a selection of recent BESIII results.
Branching fractions and the $D_{s}^{+}$ decay constant are measured using the
leptonic decays to $\mu^{+}\nu$ and $\tau^{+}\nu$. From a data sample of $0.482
fb^{-1}$ collected at the $D_{s}^{+}D_{s}^{-}$ threshold we measure
$f_{Ds}=(241 \pm 16.3(stat.) \pm 6.6(sys.)) MeV$. BESIII recently found
preliminary evidence of the decay $D_{s}^{+}\rightarrow\tau^{+}\nu$ and with a
significance larger than $4 \sigma$ using $2.81 fb^{-1}$ of data at the
$D^0\overline{D^0}$ threshold. Using the same data sample the decay
$D^0\rightarrow K^0_{S/L} \pi^0(\pi^0)$ is analysed. The branching fractions
are measured and using the $CP$ eigenstates $K_{S/L}^0\pi^0$ the $D^0$ mixing
parameter $y_{CP} = (0.98 \pm 2.43)\%$ is measured.
