  The archival spectrum of SDSS J212531.92-010745.9 shows not only the typical
signature of a PG1159 star, but also indicates the presence of a companion.
With time-series photometry of SDSS J212531.92-010745.9 during 10 nights,
spread over one month, with the T"ubingen 80cm and the G"ottingen 50cm
telescopes, the binary nature of this object has recently been proven. An
orbital period of 6.9h could be determined, and the observed light curve fitted
with the nightfall program. A comparison of the spectrum of SDSS
J212531.92-010745.9 with NLTE models further constrained the light curve
solution. We emphasize that this is the first system of this kind which will
allow a dynamical mass determination for a PG1159 star.
