...

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

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...

Fellow of CONICET, Argentina.

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... nebulae)

In this paper, naked GW Vir stars and PNNV stars will be indistinctly referred to as GW Vir stars or pulsating PG 1159.

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... stratification

In addition, pulsating PG 1159 stars have recently been shown by Córsico & Althaus (2005) to be valuable tools to constrain the occurrence of extra mixing episodes in their progenitor stars.

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... mass

These abundances are not only determined by the stellar mass, but also by the deepness of the envelope convection and mass loss episodes after the VLTP (see Miller Bertolami & Althaus 2006 for comments). Also, the number of thermal pulses considered in the AGB stage and the efficiency of overshooting play a role in the final PG 1159 surface composition (Herwig 2000).

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... periods

We note, however, that for modes with the shortest periods the value of $\eta$ is so small ($\la$10^-9) that they are only marginally unstable.

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... respectively)

Note that the helium abundance range predicted by our models (0.3-0.5) is coincident with the observed one for most of PG 1159 stars, as reported by Werner & Herwig (2006).

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... region

Note that in Figs. 3 and 4 the blue edge for $\ell = 2$ crosses the evolutionary track of $0.565~M_{\odot}$; it is a parameterization of the blue edge and not the exact theoretical blue edge emerging from our stability analysis.

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