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With the aim of increasing the sample of M31 clusters for which a colourmagnitude diagram is available, we searched the HST archive for ACS imagescontaining objects included in the Revised Bologna Catalogue of M31 globularclusters. Sixty-three such objects were found. We used the ACS images toconfirm or revise their classification and we obtained useful CMDs for 11 oldglobular clusters and 6 luminous young clusters. We obtained simultaneousestimates of the distance, reddening, and metallicity of old clusters bycomparing their observed field-decontaminated CMDs with a grid of templateclusters of the Milky Way. We estimated the age of the young clusters byfitting with theoretical isochrones. For the old clusters, we foundmetallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree withexisting spectroscopic extimates. At least four of them display a clear blueHB, indicating ages >10 Gyr. All six candidate young clusters are found to haveages <1Gyr. With the present work the total number of M31 GCs with reliableoptical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 forthe young ones. The old clusters show similar characteristics to those of theMW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6and located at a large projected distance from the centre of M31 and from thegalaxy major axis. Metal-rich globulars at large galactocentric distances arerare both in M31 and in the MW. B407, in addition, has a velocity in starkcontrast with the rotation pattern shared by the bulk of M31 clusters ofsimilar metallicity. This, along with other empirical evidence, supports thehypothesis that the cluster is physically associated with a substructure in theM31 halo that has been interpreted as the relic of a merging event.

HST/ACS colour-magnitude diagrams of M31 globular clusters