EROs found behind lensing clusters. II. Empirical properties, classification, and SED modelling based on multi-wavelength observations

International audienceAims.We study the properties and nature of extremely red galaxies (ERO, R-K?5.6) found behind two lensing clusters and compare them with other known galaxy populations. Methods: New HST/ACS, Spitzer (IRAC and MIPS), and Chandra/ACIS observations of the two lensing clusters Abell 1835 and AC114 have been obtained, combined with our earlier optical and near-IR observations and used to study EROs in these deep fields. Results: We have found 6 and 9 EROs in Abell 1835 and AC114. Several (7) of these objects are undetected up to the I and/or z850 band, and are hence ?optical? drop-out sources (at a 3? limit). The photometric redshifts of most of our sources (80%) are z? 0.7-1.5. According to simple colour-colour diagrams, the majority of our objects would be classified as hosting old stellar populations (?ellipticals?). However, there are clear signs of dusty starbursts for several among them. These objects correspond to the most extreme ones in R-K colour. After correcting for lens magnification, we estimate a surface density of (0.97±0.31) arcmin-2 for EROs with (R-K ? 5.6) at K