SIMBAD references

Combination frequencies are frequently observed in the Fourier spectra of pulsating DA and DB white dwarfs. These are powers appearing at the sum and difference frequencies of the stellar gravity-modes. Brickhill (1992) first proposed that the combination frequencies are produced when the surface convective zone changes its size during pulsation. As a certain part of the flux signal is absorbed by the convection zone, this size change distorts the originally sinusoidal flux signal and leads to the combination frequencies in the Fourier spectrum. We developed analytical expressions to describe the amplitudes and phases of combination frequencies. We apply them to one DA and one DB white dwarf and compare the results with observations. There are three free parameters in the expressions: depth of the convection zone, sensitivity of the convection zone depth to stellar effective temperature and the inclination angle between observer's line-of-sight and the white dwarf rotation axis. Reasonable values for the above parameters produce satisfactory fit to the observations. We also propose using the combinations as a tool to identify the spherical degree (l) of the gravity-modes. Moreover, it is possible to explain the combination frequencies in other pulsating stars (δ Scuti, sdB, PG 1159 stars, etc.) along the same approach.