  A measurement of the charm-mixing parameter $y_{CP}$ using $D^0 \to K^+ K^-$,
$D^0 \to \pi^+ \pi^-$, and $D^0 \to K^- \pi^+$ decays is reported. The $D^0$
mesons are required to originate from semimuonic decays of $B^-$ and
$\overline{B}^0$ mesons. These decays are partially reconstructed in a data set
of proton-proton collisions at center-of-mass energies of 7 and 8 TeV collected
with the LHCb experiment and corresponding to an integrated luminosity of 3
fb$^{-1}$. The $y_{CP}$ parameter is measured to be $(0.57 \pm 0.13(\rm{stat.})
\pm 0.09(\rm{syst.}))\%$, in agreement with, and as precise as, the current
world-average value.
