| Abstract
| - We report signal-to-noise ratio (S/N) ≃ 100 optical spectra for four DA white dwarf stars acquired with the GMOS spectrograph of the 8-m Gemini north telescope. These stars have 18 < g< 19 and are around Teff∼ 12 000 K, where the hydrogen lines are close to maximum. Our purpose is to test if the effective temperatures and surface gravities derived from the relatively low-S/N (〈S/N〉≈ 21) optical spectra acquired by the Sloan Digital Sky Survey through model atmosphere fitting are trustworthy. Our spectra range from 3800 to 6000 Å, therefore including Hβ to H9. The H8 line was only marginally present in the SDSS spectra, but is crucial to determine the gravity. When we compare the values published by Kleinman et al. and Eisenstein et al. with our line-profile technique (LPT) fits, the average differences are: ΔTeff≃ 320 K, systematically lower in the SDSS, and Δ log g≃ 0.24 dex, systematically larger in the SDSS. The correlation between the gravity and the effective temperature can only be broken at wavelengths bluer than 3800 Å. The uncertainties in Teff are 60 per cent larger, and in log g larger by a factor of 4, than the internal uncertainties of Kleinman et al. and Eisenstein et al.
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