Comparison of best-fitting model spectrum (red) with observation (black) in the case of B1085. Residuals χ are shown in the lower panels. Left figure: one of the available MMT spectra, the quality of which is typical for the sample. Right figure: exemplary flux-calibrated X-shooter UVB spectrum. Contrary to the absolute fluxes, the calibration of the relative fluxes is almost identical, demonstrating that our procedure for relative flux-calibration works. Combined with synthetic spectra that properly account for the Balmer jump, this enables us to derive reliable effective temperatures and surface gravities.