Résumé
Superconducting YBa2Cu3O6.6 samples (TC=40K) were investigated by the muon spin rotation technique in zero external field. Below 3 K we find a fast depolarization of the muon signal which indicates that local magnetic fields believed to originate from Cu electronic moments are present. For temperatures above 3 K and for higher oxygen concentrations these fields are absent. We suggest that dynamic spin correlations exist in YBa2Cu3Oy in a wide temperature and concentration range but only for y < 6.6 and low temperatures the fluctuations are sufficiently slow to be seen in a μSR experiment.