Left: SPY sample (black dots) plotted in a Hertzsprung–Russell diagram (Gaia absolute magnitude versus Gaia colour) on top of a background of 11 561 white dwarf candidates selected from the Gaia-derived sample of Gentile Fusillo et al. (2019) and colour-coded by plot density. The large blue dots on the left mark the main cooling track of the Gaia white dwarfs for G_BP − G_RP >  0. The dashed line represents the polynomial fitted to the brightness distribution. Right: histograms of the differential magnitudes for Gaia (solid grey) and SPY (outline blue).