  The absolute calibration of space-borne instruments in the soft X-ray regime
rests strongly on model spectra of hot white dwarfs.
  We analyze the Chandra LETG+HRC-S observations of the white dwarfs HZ43 A and
Sirius B and of the neutron star RX J185635-3754 in order to resolve current
uncertainties in the soft X-ray spectral fluxes and photospheric parameters of
the three stars. We have obtained improved parameters for which fit the
observations from the optical to the soft X-ray regime. Our approach allows us
to quote their absolute spectral fluxes at selected wavelengths which may aid
the calibration of other space-borne instruments.
