Absolute magnitude–colour diagram of single white dwarfs (dark grey) and main sequence stars (light grey) within 100 pc from Jiménez-Esteban et al. (2023), hot sub-dwarf stars (red dots) from Geier (2020), and ELM white dwarfs (blue dots) from Brown et al. (2016), Kilic et al. (2017), and Pelisoli et al. (2018). J0526+5934 (solid magenta dot) falls in the transition region between hot sub-dwarf stars and ELM white dwarfs. The open black circle represents the G_abs vs. Bp-Rp that we obtain for J0526+5934 from our spectral modelling (Sect. 4.2), which is in good agreement with the Gaia observed value (magenta dot). All magnitudes have been de-reddened using the extinction provided by the Lallement et al. (2014) 3D maps except for J0526+5934 (in this case, we used the reddening obtained from our analysis; Sect. 4.2), and only objects with positive parallaxes and relative errors below 10% have been considered. The cyan and green lines represent the evolutionary tracks for a 0.226 M_⊙ and a 0.324 M_⊙ ELM white dwarf, respectively, from Istrate et al. (2016).