Representative models of IM Lupi. The disc has been rotated so that the semi-major axis is horizontal. From left to right a): without any freeze-out of CO; b): with complete freeze-out of CO, where T< 21 K; c): with depletion of gaseous CO by 10^-4, where T< 21 K; d): same as c) including the interstellar radiation field. The models are designed for qualitative comparison only. They are convolved to the ALMA resolution, but not degraded to the same noise level as the observations.