CMD for all sources in the M 87 SUB1 (top panel) and M 87 SUB2 (bottom panel) fields. The horizontal lines indicate the colour excess of emission line objects with an EW_obs = 110 Å. The curved lines delimit the regions above which 99% and 99.9% of the simulated continuum objects fall in this diagram, given the photometric errors. The set of points on the inclined line represents those objects with no broadband magnitude measured by SExtractor (see Sect. 3.1 for more details). Asterisks represent objects classified as PNs according to the selection criteria discussed in Sect. 3.1.