Correlations between L/M and the abundances of o-H₂D⁺, N₂D⁺, and N₂H⁺ obtained with the LTE analysis. Blue dots refer to sources associated with the evolutionary sequence defined in Sect. 2.2, whilst grey dots are associated with PDRs. Shaded dots refer to sources in which o-H₂D⁺ is not detected. The blue dashed lines show the result of a linear least-squares fit to the data, while the cyan shaded areas show the 3σ uncertainties on the best-fit parameters (errors in parenthesis).