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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 galaxypopulations. New HST/ACS observations, Spitzer IRAC and MIPS, and Chandra/ACISobservations of the two lensing clusters Abell 1835 and AC114 contemplate ourearlier optical and near-IR observations and have been used to study extremelyred objects (EROs) in these deep fields. We have found 6 and 9 EROs in Abell1835 and AC114. Several (7) of these objects are undetected up to the I and/orz band, and are hence ``optical'' drop-out sources. The photometric redshiftsof most of our sources (80%) are z~0.7-1.5. According to simple colour-colourdiagrams the majority of our objects would be classified as hosting old stellarpopulations. However, there are clear signs of dusty starbursts for severalamong them. These objects correspond to the most extreme ones in R-K colour. Weestimate a surface density of (0.97+-0.31) arcmin-2 for EROs with (R-K>5.6) atK<20.5. Among our 15 EROs 6 (40 %) also classify as distant red galaxies(DRGs). 11 of 13 EROs with available IRAC photometry also fulfil the selectioncriteria for IRAC selected EROs (IEROs) of Yan et al. (2004). SED modellingshows that ~ 36 % of the IEROs in our sample are luminous or ultra-luminousinfrared galaxies ((U)LIRG). Some very red DRGs are found to be very dustystarbursts, even (U)LIRGs, as also supported by their mid-IR photometry. Noindication for AGNs is found, although faint activity cannot be excluded forall objects. From mid-IR and X-ray data 5 objects are clearly classified asstarbursts. The derived properties are quite similar to those of DRGs andIEROs, except for 5 extreme objects in terms of colours, for which a very highextinction (Av>3) is found.

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