  Combination frequencies are observed in the Fourier spectra of pulsating DA
and DB white dwarfs. They appear at sums and differences of frequencies
associated with the stellar gravity-modes. Brickhill (1992) proposed that the
combination frequencies result from mixing of the eigenmode signals as the
surface convection zone varying in depth when undergoing pulsation. This depth
changes cause time-dependent thermal impedance, which mix different harmonic
frequencies in the light curve.
  Following Brickhill's proposal, we developed analytical expressions to
describe the amplitudes and phases of these combination frequencies. The
parameters that appear in these expressions are: the depth of the stellar
convection zone when at rest, the sensitivity of this depth towards changes in
stellar effective temperature, the inclination angle of the stellar pulsation
axis with respect to the line of sight, and lastly, the spherical degrees of
the eigenmodes involved in the mixing. Adopting reasonable values for these
parameters, we apply our expressions to a DA and a DB variable white dwarf. We
find reasonable agreement between theory and observation, though some
discrepancies remain unexplained. We show that it is possible to identify the
spherical degrees of the pulsation modes using the combination frequencies.
