WD sequence (grey points) in the Gaia Hertzsprung-Russell diagram. Theoretical ONe-core WD cooling tracks of 1.05 and 1.29 M_⊙ disregarding ²²Ne diffusion are shown as solid blue lines (Camisassa et al. 2019). The Q branch is represented as an observed overabundance of WDs below the standard cooling sequence. In order to mark off the Q branch, we have defined the parameter ζ = M_G − 1.2 × (G_BP − G_RP). The ultramassive Q branch is delimited by ζ = 13.4 and ζ = 13.0, and by the cooling tracks of 1.05 and 1.29 M_⊙. Dashed red lines delimit the region where we have counted WDs to prepare the histograms.