Two neutrino double beta decay within the ξ-approximation

We examine the contributions of odd-parity nuclear operators to the two-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 αZ/2 or v/c of the nucleon than the usual Fermi and Gamow-Teller matrix elements, explicit calculations performed within the QRPA show that they are significant when compared with the experimental data.