It is thought that the atmospheric composition of PG 1159 stars is odd because, after they have left the
asymptotic giant branch, they have reignited helium
fusion. As a result, a PG 1159 star's atmosphere is a mixture of material which was between the hydrogen- and helium-burning shells of its AGB star progenitor.[3], §1. They are believed to eventually lose mass, cool, and become
DO white dwarfs.[2]; [5], §4.
Some PG 1159 stars have varying
luminosities. These stars vary slightly (5–10%) in brightness due to non-radial
gravity wave pulsations within themselves. They vibrate in a number of
modes simultaneously, with typical periods between 300 and 3,000
seconds.[6][7], Table 1. The first known star of this type is also PG 1159-035, which was found to be variable in 1979,[8] and was given the variable star designation GW Vir in 1985.[9] These stars are called GW Vir stars, after their prototype, or the class may be split into DOV and PNNV stars.[7], § 1.1;[10]
^Asteroseismology of white dwarf stars, D. E. Winget, Journal of Physics: Condensed Matter10, #49 (December 14, 1998), pp. 11247–11261. DOI 10.1088/0953-8984/10/49/014.
^PG1159-035: A new, hot, non-DA pulsating degenerate, J. T. McGraw,
S. G. Starrfield, J. Liebert, and R. F. Green, pp. 377–381 in White Dwarfs and Variable Degenerate Stars, IAU Colloquium #53, ed. H. M. van Horn and V. Weidemann, Rochester: University of Rochester Press, 1979.
^The 67th Name-List of Variable Stars, P. N. Kholopov, N. N. Samus, E. V. Kazarovets, and N. B. Perova, Information Bulletin on Variable Stars, #2681, March 8, 1985.