A COOL PG-1159 STAR DISCOVERED BY THE HAMBURG SCHMIDT SURVEY - NLTE ANALYSIS OF HS-0704+6153

The discovery of a new PG 1159 star by follow-up spectroscopy of candi dates selected from objective-prism plates of the Hamburg Schmidt Surv ey is presented. From a NLTE model atmosphere analysis an effective te mperature of 65 000 K and a surface gravity of log g=7.0 is determined , which together with the extreme helium abundance (H/He< 1 by number) place HS 0704+6153 in the region of hot DO white dwarfs. The high car bon and oxygen abundances (C/He=0.2, O/He approximately 0.05 by number ), however, are typical for PG 1159 stars. Its effective temperature i s by far lower than that of any other PG 1159 star. HS 0704+6153 there fore is in the transition stage between the PG 1 159 stars and the DO white dwarfs, it hence is an ideal object to study the gravitational s ettling of metals in helium-rich atmospheres.