Context. High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG 1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). Aims: The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG 1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods: The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results: No metals could be identified in LB 1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD 246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG 1520+525 constrains the effective temperature to Teff = 150 000 ± 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GW Vir class (PG 1159 - 035) defines the location of the blue edge of the GW Vir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions: Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG 1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.
