Logarithm of C₂H (N = 3– 2, J = 7∕2–5∕2, F = 4–3 and F = 3–2) integrated line luminosity of nine Lupus protoplanetary discs vs. the logarithm of ¹³CO (J = 3– 2) integrated line flux over the continuum total flux density at 890 μm (integrated over a band width of 1 km s⁻¹ for convenience). The solid grey line shows the typical sensitivity obtained at the distance of our lowest detection (Sz129, at 162 pc).