This star is of great interest since it may well define the blue edge of the ZZ Ceti instability strip, which depends strongly on the a-priori unknown parametrization of the MLT (e.g., Bergeron et al. 1992). G 226-29 was observed with the HST using the FOS. Five observing runs were made, each about three hours long. The individual exposure times were about 9 s. Figure 4a shows the average spectra of the individual runs (these are corrected for a wavelength-dependent light loss modulated with the HST orbital period). Near about 1500 Å differences between the runs appear. However, the spectrum averaged over all runs (Fig. 4b) can be assumed to not be affected by this (it is also hardly affected by the light loss). We therefore fit this spectrum and obtain an average (the error is dominated by systematic errors which are hard to estimate) at (the latter adopted from Koester et al. 1996) using the MLT, , l=1 and . The latter two parameters were adopted from results of WET observations by Kepler et al. (1995). These observations showed one triplet in the Fourier spectrum. It should be noted, however, that this interpretation of the triplet is not the only one possible. We exclude in the fit the region above about 2300 Å where the observed slope changes. This is evident also in other HST spectra and indicates some calibration problem. The high signal-to-noise HST spectrum reveals differences between observations and theory. These could be caused by continuing calibration problems, an imperfect line broadening theory for , an imperfect description of convection or a combination of those. Finally, we fit in Figs. 4c and d the average maximum and minimum spectra. A good fit is obtained indicating that varies on the WD surface by up to .
Figure: HST spectra of G 226-29. (a) average spectra of the 5 runs corrected for light losses. (b) spectrum averaged over all runs. (c) spectrum averaged at phases near maxima. (d) spectrum averaged at phases near minima. In (b) to (d) a fit is overplotted (dotted) using , , , l=1, and an amplitude such that