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.
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