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tokens: observations of the six targets were obtained between 2016 and 2020 using a variety of instruments: X-shooter on the Very Large Telescope (VLT; Vernet et al. 2011 ) observatory at Cerro Paranal, Chile; the Intermediate-dispersion Spectrograph and Imaging System 1 (ISIS) on the William Herschel Telescope (WHT) at Roque de los Muchachos Observatory (ORM) in La Palma, Spain; the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS; Sánchez et al. 2012 ) on the Gran Telescopio Canarias (GTC) at the ORM; the High Resolution Echelle Spectrometer (HIRES; Vogt et al. 1994 ) on the 10-m Keck I telescope at Mauna Kea Observatory, Hawaii; the Magellan Echellette (MagE) spectrograph on the 6.5-m Baade Magellan telescope at Las Campanas Observatory, Chile; and the FAST spectrograph on the 1.5-m Fred Lawrence Whipple Observatory telescope on Mount Hopkins, Arizona (Fabricant et al. 1998 ). The observation log is reported in Table 1 , and sample spectra are displayed in Figs A1 and A2 in the online appendix. Table 1. Log of spectroscopic observations. X-shooter observations were carried out using a 1-arcsec slit aperture for the ultraviolet (UV)-blue (UVB) arm and 0.9 arcsec for the visible (VIS) arm. In all cases flux in the NIR arm was insufficient for any meaningful analysis. On average the resolving power of our X-shooter spectra is R ≃ 4000–5500 in the UVB arm and R ≃ 6500–8000 in the VIS arm. All data were reduced using the standard procedures within the reflex 2 reduction tool developed by ESO. Telluric lines removal was performed on the reduced spectra using molecfit (Kausch et al. 2015 ; Smette et al. 2015 ). ISIS observations were obtained using the R600B and R600R gratings, in the ISIS blue and red arms, respectively; two central wavelength settings were used for each arm, nominally 3930 and 4540 Å in the blue and 6562 and 8200 Å in the red, which were chosen to cover important atmospheric lines, such as the Balmer series and Ca HK, as well as the Ca ii emission triplet in addition to other potential emission features (e.g. Fe ii /Mg i 5175 Å). The slit width varied between 1 and 1.5 arcsec depending on the observing conditions and we employed a binning of 2 × 2, resulting in an average resolution of ≈2 Å. ISIS spectra were debiased and flat-fielded using the standard starlink 3 packages kappa, figaro , and convert . We carried out optimal spectral extraction using the package pamela . Wavelength and flux calibration were performed using the routines within the software molly 4 (Marsh 1989 ). The OSIRIS spectroscopy of WD J0846+5703 was obtained using the R2500I grism with a 0.6-arcsec slit, and binning the detector 2 × 2. This set-up provides wavelength coverage over Å at a resolution of R ≃ 2500. The data were bias and flat-field corrected using the standard tools within iraf . 5 We performed sky background subtraction and optimal spectral extraction (Horne 1986 ) using the pamela data reduction