2 NEUTRINO DOUBLE-BETA DECAY WITHIN THE XI-APPROXIMATION

We examine the contributions of odd-parity nuclear operators to the tw o-neutrino double beta decay 0(+) --> 0(+) amplitude, which come from the P-wave coulomb corrections to the electron wave functions and the recoil corrections to the nuclear currents. Although they are formally of higher order in alpha Z/2 or upsilon/c of the nucleon than the usu al Fermi and Gamow-Teller matrix elements, explicit calculations perfo rmed within the QRPA show that they are significant when compared with the experimental data.