  Adding Au nanoparticles to YBa$_2$Cu$_3$O$_{7-\delta}$ thin films leads to an
increase of the superconducting transition temperature $T_{\rm c}$ and the
critical current density $j_{\rm c}$. While the higher $j_{\rm c}$ can be
understood in terms of a stronger pinning of the flux vortices at the Au
nanoparticles, the enhanced $T_{\rm c}$ is still puzzling. In the present
study, we determined the microscopic magnetic penetration profiles and the
corresponding London penetration depths $\lambda_{\rm L}$ in the Meissner state
of optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ thin films with and without Au
nanoparticles by low-energy muon spin rotation. By Rutherford backscattering
spectrometry, we show that the Au nanoparitcles are distributed over the whole
thickness of the thin-film samples. The superfluid density $n_{\rm s} \propto
1/\lambda_{\rm L}^2$ was found to increase in the films containing Au
nanoparticles. We attribute this increase of $n_{\rm s}$ to a reduction of the
defect density possibly due to defect condensation at the Au nanoparticles.
