  We simulate a theory with two dynamical O($a$) improved Wilson quarks whose
mass $M$ ranges from a factor eight up to a factor two below the charm quark
mass and at three values of the lattice spacing ranging from 0.066 to 0.034 fm.
This theory is a prototype to study the decoupling of heavy quarks. We measure
the mass and cut-off dependence of ratios of gluonic observables defined from
the Wilson flow or the static potential. The size of the 1/$M$ corrections can
be determined and disentangled from the lattice artifacts. The difference with
the pure gauge theory is at the percent level when two quarks with a mass of
the charm quark are present.
