  We consider effects of coloured scalar mediators in decays $c\to u \, {\it
invisibles}$. In particular, in these processes, as invisibles, we consider
massive right-handed fermions. The coloured scalar $\bar S_1\equiv (\bar 3, 1,
-2/3)$, due to its coupling to weak singlets up-quarks and invisible
right-handed fermions ($\chi$), is particularly interesting. Then, we consider
$\tilde R_2 \equiv (\bar 3, 2, 1/6)$, which as a weak doublet is a subject of
severe low-energy constraints. The $\chi$ mass is considered in the range $(m_K
- m_\pi)/2\leq m_\chi \leq (m_D - m_\pi)/2$. We determine branching ratios for
$D\to \chi \bar \chi$, $D\to \chi \bar \chi \gamma$ and $D\to \pi \chi \chi$
for several $\chi$ masses, using most constraining bounds. For $\bar S_1$, the
most constraining is $D^0 -\bar D^0$ mixing, while in the case of $\tilde R_2$
the strongest constraint comes from $B\to K {\it missing\, energy}$ . We find
in decays mediated by $\bar S_1$ that branching ratios can be $\mathcal B(D\to
\chi \bar \chi)< 10^{-8}$ for $m_\chi=0.8$ GeV, $\mathcal B(D\to \chi \bar \chi
\gamma) \sim 10^{-8}$ for $m_\chi=0.18$ GeV, while $\mathcal B(D^+ \to \pi^+
\chi \bar \chi )$ can reach $ \sim 10^{-8}$ for $m_\chi=0.18$ GeV. In the case
of $\tilde R_2$ these decay rates are very suppressed. We find that future
tau-charm factories and Belle II experiments offer good opportunities to search
for such processes. Both $\bar S_1$ and $\tilde R_2$ might have masses within
LHC reach.
