Simulated C₂H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc dust mass compared with the Lupus observations. The dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and a gas-to-dust ratio of 100 (same as in panel a of Fig. 6). The solid orange line shows model results obtained decreasing the gas masses by two orders of magnitude, while keeping the dust mass fixed in order to obtain a gas-to-dust ratio of 1. Lupus detections are shown by the black circles, and non-detections by the empty triangles as 3-σ upper limits.