  The width difference $\Delta \Gamma$ among the two mass eigenstates of the
$B_s$-$\bar B_s$ system is measured with a precision of 7%. The theory
prediction has a larger uncertainty which mainly stems from unknown
perturbative higher-order QCD corrections. I discuss the subset of
next-to-next-to-leading order diagrams proportional to $\alpha_s^2\, N_f$,
where $N_f=5$ is the number of quark flavours. The results are published in
[1].
