Simulated C₂H integrated line luminosities using the code DALI (Bruderer et al. 2012) as functions of the disc gas masses compared with the Lupus observations. Panel a: dotted black line shows model results obtained assuming [C] and [O] ISM-like abundances and [C]/[O] = 0.3. The Lupus detections are shown by the black circles and non-detections by the empty triangles as 3-σ upper limits. Panel b: solid blue line shows model results obtained assuming that [C] and [O] abundances are depleted by two orders of magnitude and [C]/[O] = 1.5. Orange, light green, green, and blue squares show the model predictions when elemental [C] and [O] are depleted by two orders of magnitude and volatile [C]/[O] = 0.3, 0.8, 1.0, and 1.5 for M_disc = 10⁻²M_⊙. The black circles and empty triangles show C₂H observationsas a function of CO-based gas masses multiplied by a factor of 100 to account for the carbon depletion.