We propose a TF-muSR investigation of the superconducting properties of the REFe_(1-y)Ru_yAsO_{1-x}F_x compound (RE=La, Sm). In this system the substitution of Fe by the isoelectronic Ru is not expected to change the doping of the FeAs layers. However we observe that the Ru substitution reduces the superconducting transition from 45K (y=0) to 10K (y=0.3). This experiment is aimed at investigating the change of the microscopic properties of the superconducting state, via the analysis of the muon linewidth, in the range 0<y<0.4 for x=0.15. It will help in elucidating the role of the d-bands of the FeAs layers in the superconducting mechanism.