  We determine the nature, redshift, stellar populations and dust properties of
optically faint or non-detected extremely red objects (ERO) found from our
survey of the lensing clusters A1835 and AC114. We perform SED fitting and use
deep optical, HST, VLT, Spitzer data, and for some objects and sub-mm data. For
most of the lensed EROs we find photometric redshifts showing a strong
degeneracy between "low-z" (z~1-3) and high-z (z~6-7). Although formally best
fits are often found at high-z, their resulting bright absolute magnitudes, the
number density of these objects, and in some cases Spitzer photometry or longer
wavelength observations, suggest strongly that all of these objects are at
"low-z". The majority of these objects are best fitted with young (<~ 0.5-0.7
Gyr) and dusty starbursts. Indications for strong extinction, with A_V~2.4-4,
are found in some objects. For dusty objects star formation rates (SFR) have
been estimated from the bolometric luminosity determined after fitting of
semi-empirical starburst, ERO, and ULIRG templates. Typically we find
SFR~(1-18) Msun/yr. Again, SMMJ14009+0252 stands out as a LIRG with SFR 1000
Msun/yr. Concerning the comparison objects, we argue that the massive
post-starburst z~6.5 galaxy candidate HUDF-J2 showing observed properties very
similar to our EROs, is more likely a dusty starburst at z~2.3-2.6. This
interpretation also naturally explains the observed 24 micron emission. Both
empirically and from our SED fits we find that the IRAC selectec EROs from Yan
et al (2004) show very similar properties to our lensed EROs. Reasonable fits
are found for most of them with relatively young and dusty stellar populations.
