Letter to the Editor

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

¹ Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: nagel@astro.uni-tuebingen.de
² Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Department of Physics, University of Warwick, Coventry, CV4 7AL, Great Britain
⁴ Departamento de Fisica y Meteorologia, Facultad de Ciencias, Universidad de Valparaiso, Valparaiso, Chile

Received: 14 December 2005
Accepted: 19 January 2006

Abstract

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 ~ 90 000 K,  ~ 7.60, and the abundance ratio C/He ~ 0.05 by number fraction. For the companion we obtained with a mean radius of 0.4 ± , a mass of 0.4 ± , 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.