Context :

Aims : We study the properties and nature of extremely red galaxies ( ERO , R - K \geq 5.6 ) found behind two lensing clusters and compare them with other known galaxy populations .

Methods : New HST/ACS observations , Spitzer IRAC and MIPS , and Chandra/ACIS observations of the two lensing clusters Abell 1835 and AC114 contemplate our earlier optical and near-IR observations ( Richard et al .
2006 ) and have been used to study extremely red objects ( 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 z _ { 850 } band , and are hence “ optical ” drop-out sources .
The photometric redshifts of most of our sources ( 80 % ) are z \sim 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 .
We estimate a surface density of ( 0.97 \pm 0.31 ) arcmin ^ { -2 } for EROs with ( R - K \geq 5.6 ) at K < 20.5 .
Among our 15 EROs 6 ( 40 % ) also classify as distant red galaxies ( DRGs ) .
11 of 13 EROs ( 85 % ) with available IRAC photometry also fulfil the selection criteria for IRAC selected EROs ( IEROs ) of Yan et al .
( 2004 ) .
SED modelling shows that \sim 36 % of the IEROs in our sample are luminous or ultra-luminous infrared galaxies ( ( U ) LIRG ) .
Some very red DRGs are found to be very dusty starbursts , even ( U ) LIRGs , as also supported by their mid-IR photometry .
No indication for AGNs is found , although faint activity can not be excluded for all objects .
From mid-IR and X-ray data 5 objects are clearly classified as starbursts .
The derived properties are quite similar to those of DRGs and IEROs , except for 5 extreme objects in terms of colours , for which a very high extinction ( A _ { V } \ga 3 ) is found .

Conclusions :