Abstract

Methods to measure masses of PG1159 stars in order to test evolutionary scenarios are currently based on spectroscopic masses or asteroseismological mass determinations. One recently discovered PG1159 star in a close binary system may finally allow the first dynamical mass determination, which has so far been analysed on the basis of one SDSS spectrum and photometric monitoring. In order to be able to phase future radial velocity measurements of the system SDSS J212531.92010745.9, we follow up on the photometric monitoring of this system to provide a solid observational basis for an improved orbital ephemeris determination. New white-light time series of the brightness variation of SDSS J212531.92010745.9 with the Tübingen 80 cm and Göttingen 50 cm telescopes extend the monitoring into a second season (2006), tripling the length of overall observational data available, and significantly increasing the time base covered. We give the ephemeris for the orbital motion of the system, based on a sine fit which now results in a period of 6.95573(5) h, and discuss the associated new amplitude determination in the context of the phased light curve variation profile. The accuracy of the ephemeris has been improved by more than one order of magnitude compared to that previously published for 2005 alone. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.