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We present deep (V ~ 28.0) BV photometry obtained with the wide field channelof the Advanced Camera for Surveys on board HST for four M31 globular clustersthat were identified as candidate intermediate-age (age ~ 1-9 Gyr) by variousauthors, based on their integrated spectra and/or broad/intermediate-bandcolors. Two of them (B292 and B350) display an obvious blue horizontal branch,indicating that they are as old as the oldest Galactic globulars. On the otherhand, for the other two (B058 and B337), which display red horizontal branches,it was not possible either to confirm or disconfirm the age estimate fromintegrated spectra. The analysis of the distribution in the spectral indicesMg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can beclassified as red or blue based on existing CMDs, strongly suggests thatclassical age diagnostics from integrated spectra may be significantlyinfluenced by the HB morphology of the clusters and can lead to erroneousage-classifications. We also provide the CMD for another two clusters that fallinto the field of the main targets, B336, an old and metal-poor globular with asignificant population of RR-Lyrae variables, and the newly discovered B531, acluster with a very red red giant branch.

HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters. The role of blue horizontal branches