Context :

Aims : The archival spectrum of SDSS J212531.92 - 010745.9 shows not only the typical signature of a PG 1159 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 Tübingen 80 cm and the Göttingen 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 PG 1159 star we found , as preliminary results from comparison with our NLTE models , T _ { eff } \sim 90 000 K , \log g \sim 7.60 , and the abundance ratio C/He \sim 0.05 by number fraction .
For the companion we obtained with a mean radius of 0.4 \pm 0.1 R _ { \odot } , a mass of 0.4 \pm 0.1 M _ { \odot } , and a temperature of 8 200 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 .

Conclusions :