U-λ colors as function of T_eff. The red circles are objects with measured U-band fluxes, arrows are objects with U-band upper limits. The squares are model-predicted colors for different gravity, from 5.5 (black squares, top) to 3.5 (blue squares, lowest). The solid lines show the best-fit relations for Class III derived in Sect. 4.1, extrapolated to lower T_eff; dashed lines are  ± 2σ. We will consider as accretors objects with colors below the photospheric strip: five BDs have clear evidence of U-band excess (note that for two we only have two colors), one only marginal evidence (from (U − I) and (U − J), while (U − Z) is photospheric). Hereafter we mark the upper limit as dashes for clarity. (A color version of this figure is available in the online journal.)