It has been known for a long time that white dwarfs are pulsationally unstable if nuclear burning takes place in their envelopes. Perturbation of energy generation rate promotes pulsational instability and this effect is frequently referred to as ε-mechanism. In recent years, with the advent of high-speed photometry, many rapidly varying white dwarfs have been discovered. However, periods of variability were found to be significantly longer than the periods of radial pulsations which were the only type of oscillations considered before the discovery. Furthermore, the case of ε-mechanism as being responsible for the observed variability has never been made strong for any of the observed objects.Variable white dwarfs are found among: Io single DA-type objects in the effective temperature range 10000-15000K; 2o members of close, usually but not always, cataclysmic binary systems. Although, following an early suggestion by Warner and Robinson (1972), the excitation of nonradial oscillation is postulated in both cases, the two types represent very different physical situations and they will be discussed here separately.

The rich oscillation spectra determined for the two stars, ν Eridani and 12 Lacertae, present an interesting challenge to stellar modelling. The stars are hybrid objects showing not only a number of modes at frequencies typical for β Cep stars but also one mode at frequency typical for SPB stars. We construct seismic models of these stars considering uncertainties in opacity and element distribution. We also present estimate of the interior rotation rate and address the matter of mode excitation.We use both the opacity project (OP) and opacity library Livermore (OPAL) opacity data and find significant difference in the results. Uncertainty in these data remains a major obstacle in precise modelling of the objects and, in particular, in estimating the overshooting distance. We find evidence for significant rotation rate increase between envelope and core in the two stars.Instability of low-frequency g modes was found in seismic models of ν Eri built with the OP data, but at frequencies higher than those measured in the star. No such instability was found in models of 12 Lac. We do not yet have a satisfactory explanation for the low-frequency modes. Some enhancement of opacity in the driving zone is required but we argue that it cannot be achieved by the iron accumulation, as it has been proposed.

Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.

Abstract. In δ Scuti star models, the calculated amplitude ratios and phase differences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for the determination of the spherical harmonic degree, , of the excited modes. The dependence on convection enters through the complex parameter f , which describes bolometric flux perturbation. We present a method of simultaneous determination of f and harmonic degree from multi-colour data and apply it to three δ Scuti stars. The method indeed works. Determination of appears unique and the inferred values of f are sufficiently accurate to yield a useful constraint on models of stellar convection. Furthermore, the method helps to refine stellar parameters, especially if the identified mode is radial.

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Data on multimode Cepheids from OGLE-III catalog of the LMC Cepheids are confronted with results from model calculations. Models whose radial mode periods are consistent with observation are not always in agreement with published evolutionary models. Nonradial mode interpretation is considered for the cases of unusual period ratios. The greatest challenge for stellar pulsation theory is explanation of double-mode pulsators with period ratios near 0.6.