Constraints on the ratio of flux density originating from the population of larger grains with the grain radius a_L = 10 µm (gold), a_L = 100 µm (orange), and a_L = 1000 µm (dark red) to that originating from the population of smaller grains with any possible grain radius a_S for the four observing wavelengths 2.13 µm, 4.1 µm, 11.1 µm, and 870 µm. The horizontal dashed red lines denote equal contribution of the two grain populations to the total flux density (i.e., ). Constraints are computed without taking observational uncertainties into account, except for the systems HD 10700, HD 187642, and HD 203280 for which no other constraints could be derived (see Sects. 3.1 and 3.4).