  We study the sub-mm properties of color-selected galaxies via a stacking
analysis applied for the first time to interferometric data at sub-mm
wavelengths. We base our study on 344 GHz ALMA continuum observations of
~20''-wide fields centered on 86 sub-mm sources detected in the LABOCA Extended
Chandra Deep Field South Sub-mm Survey (LESS). We select various classes of
galaxies (K-selected, star-forming sBzK galaxies, extremely red objects and
distant red galaxies) according to their optical/NIR fluxes. We find clear,
>10-sigma detections in the stacked images of all these galaxy classes. We
include in our stacking analysis Herschel/SPIRE data to constrain the dust SED
of these galaxies. We find that their dust emission is well described by a
modified black body with Tdust~30 K and beta=1.6 and IR luminosities of
(5-11)x10^{11} Lsun, or implied star formation rates of 75-140 Msun/yr. We
compare our results with those of previous studies based on single-dish
observations at 870 micron and find that our flux densities are a factor 2-3
higher than previous estimates. The discrepancy is observed also after removing
sources individually detected in ALESS maps. We report a similar discrepancy by
repeating our analysis on 1.4\,GHz observations of the whole ECDFS. Hence we
find tentative evidence that galaxies that are associated in projected and
redshift space with sub-mm bright sources are brighter than the average
population. Finally, we put our findings in the context of the cosmic star
formation rate density as a function of redshift.
