SDSS J212531.92-010745.9 - the first definite PG 1159 close binary system

Aims. 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. Our aim was the proof of the binary nature of this object and the determination of its orbital period. Methods. We performed time-series photometry of SDSS J212531.92-010745.9. We observed the object during 10 nights, spread over one month, with the Tubingen 80 cm and the Gottingen 50 cm telescopes. We fitted the observed light curve with a sine and simulated the light curve of this system with the nightfall program. Furthermore, we compared the spectrum of SDSS J212531.92-010745.9 with NLTE models, the results of which also constrain the light curve solution. Results. An orbital period of 6.95616(33) h with an amplitude of 0.354(3) mag is derived from our observations. A pulsation period could not be detected. For the PG1159 star we found, as preliminary results from comparison with our NLTE models, T-eff similar to 90 000 K, log g similar to 7.60, and the abundance ratio C C/He similar to 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4 +/- 0.1 R-., amass of 0.4 +/- 0.1 M-., and a temperature of 8200 K on the irradiated side, good agreement between the observed light curve and the, nightfall simulation, but we do not regard those values as final.