...1,2,

Member of the Carrera del Investigador Científico y Tecnológico, CONICET, Argentina.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...

Fellow of CONICET, Argentina.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... abundances

Throughout this paper, we shall use abundances by mass fraction.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... scenario

The spread in surface composition of most PG 1159 stars can be explained by differences in stellar mass and the number of thermal pulses experienced by progenitors during the AGB.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...$\sim$0.75

During subsequent thermal pulses, the helium abundance in the intershell layer gradually decreases if convective extramixing like overshooting is considered; see Herwig (2000).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... star

We are performing new simulations that show that tracks for H and He burning post-AGB stars are remarkably different for low-mass remnants (M<0.55 $M_{\odot }$).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... nebula

We note that the kinematical age of the nebula is not expected to change appreciably as a result of our lower stellar mass for K1-27.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.