Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.

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

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... Benvenuto

Member of the Carrera del Investigador Científico, CIC, Argentina.

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... Serenelli

Fellow of CONICET, Argentina.

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... phenomenon

Auther physically plausible mechanism of mode filtering is related to the interaction of convection with pulsation (see Gautschy et al. 1996).

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... lines

Strictly speaking, $\Delta P_{\rm A}$ as given by Eq. (3) corresponds to the asymptotic (high k) separation of periods for chemically homogeneous and radiative stars. However, it is very close to the mean period spacing even for chemically stratified white dwarfs. See Tassoul et al. (1990) for details.

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... $M_{\rm H} = 10^{-4}~M_*$ )

Note that, as found by Brassard et al. (1992a), the contrast between the kinetic energies values of the trapped modes and those of the non-trapped ones is not very large for massive hydrogen envelopes (like that presented in this work), due to the fact that the hydrogen-helium interface is located in a deep, highly degenerate region, where the amplitudes of eigenfunctions are very small.

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