Selection of the OBA sample. Left and right panels: the OBA samples from ESP-HS and GSP-Phot, respectively. A first filter on the parallax (S/N) is applied to the GSP-Phot targets (from panels a to c). For both samples, subluminous targets are removed (grey shading in panels b and c), and then the outliers at six standard deviations from the expected colour vs. T_eff relation (blue line) are filtered out (grey shading in panels d and e). The absolute magnitude M_G is computed using the measured parallax and the estimated interstellar extinction A_G provided by both modules. The de-reddened colour, (G_BP − G_RP)₀, is derived using the value of E(G_BP − G_RP). The resulting Kiel diagrams are shown in the bottom rows (panels f and g). The over-densities seen at T_eff = 15 000 K, 20 000 K, and 30 000 K are linked to the temperature limits of the adopted synthetic spectra libraries.