  The evolutionary scenarios which are commonly accepted for PG 1159 stars are
mainly based on numerical simulations, which have to be tested and calibrated
with real objects with known stellar parameters. One of the most crucial but
also quite uncertain parameters is the stellar mass. PG 1159 stars have masses
between 0.5 and 0.8 M_sun, as derived from asteroseismic and spectroscopic
determinations. Such mass determinations are, however, themselves
model-dependent. Moreover, asteroseismically and spectroscopically determined
masses deviate systematically for PG 1159 stars by up to 10%. SDSS
J212531.92-010745.9 is the first known PG 1159 star in a close binary with a
late-main-sequence companion allowing a dynamical mass determination. We have
obtained 14 Calar Alto spectra of SDSS J212531.92-010745.9 covering the full
orbital phase range. A radial velocity curve was extracted for both components.
With co-added phase-corrected spectra the spectral analysis of the PG 1159
component was refined. The irradiation of the companion by the PG 1159 star is
modelled with PHOENIX, yielding constraints on radii, effective temperature and
separation of the system's components. The light curve of SDSS
J212531.92-010745.9, obtained during three seasons of photometry with the
Goettingen 50 cm and Tuebingen 80 cm telescopes, was modelled with both the
NIGHTFALL and PHOEBE programs.
