Fractional dust luminosities L_dust/L_∗ for the DUNES_DU and DUNES_DB stars with excesses (squares) and d ≤ 20 pc (Table 14 in E13 and Table 2 in this work), and upper limits for the non-excess sources (crosses), all of them plotted against metallicity, [Fe/H]. F, G, and K stars are plotted in violet, green, and red. The short-dashed vertical line marks the mean metal abundance of the whole sample, [Fe/H] = −0.11, whereas the long-dashed line marks the median, [Fe/H] = −0.095. The horizontal lines mark the means (solid) of the fractional luminosities for the debris discs of stars showing excess, with [Fe / H] ≤ −0.11 and >−0.11, and plus and minus their standard errors (dotted), defined as σ/, where σ is the standard deviation; these computations have been done over the values of log (L_dust/L_∗).