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

Abstract

\n Aims.The archival spectrum of SDSS J212531.92-010745.9 shows not\n only the typical signature of a PG 1159 star, but also indicates the\n presence of a companion. Our aim was the proof of the binary nature of\n this object and the determination of its orbital period.Methods.We performed time-series photometry of SDSS J212531.92-010745.9. We\n observed the object during 10 nights, spread over one month, with the \n Tübingen 80 cm and the Göttingen 50 cm telescopes. We fitted the\n observed light curve with a sine and simulated the light curve of this\n system with the <monospace>nightfall</monospace> program. Furthermore, we compared the\n spectrum of SDSS J212531.92-010745.9 with NLTE models, the results of which \n 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\n PG 1159 star we found, as preliminary results from comparison with our NLTE models, Teff ~ 90 000 K, $\\log g$ ~ 7.60, and the abundance ratio\n C/He ~ 0.05 by number fraction. For the companion we obtained with a mean radius of\n 0.4 ± $0.1~R_\\odot$, a mass of 0.4 ± $0.1~M_\\odot$, and a temperature of 8200 K on the irradiated side, good agreement between\n the observed light curve and the <monospace>nightfall</monospace> simulation, but we\n do not regard those values as final. \n