  We study the effect of deformation on the two-neutrino double beta decay for
ground state to ground state transitions in all the nuclei whose half-lives
have been measured. Our theoretical framework is a deformed QRPA based in
Woods-Saxon or Hartree-Fock mean fields. We are able to reproduce at the same
time the main characteristics of the two single beta branches, as well as the
double beta matrix elements. We find a suppression of the double beta matrix
element with respect to the spherical case when the parent and daughter nuclei
have different deformations.
