Dust optical depths, τ_ν, as a function of the SED temperature at 333 GHz (red lines) and at 246.2 GHz (green lines) for each source. The shaded grey area corresponds to the range of temperature for which the β values derived are authorised. The full range of optical depth values derived over the temperature range for each source is indicated on the top right side of the plot. For single systems, the bolometric temperature, T_bol, derived from Herschel (Furlan et al. 2016) is indicated by a blue dashed line and the radiative temperature, T_rad, by a red dashed area or by a red dashed line in the case of FIR2. Black arrows indicate that the derived values are lower limits.