  The one-QRPA method is used to describe simultaneously both double decay beta
modes, giving special attention to the partial restoration of spin-isospin
SU(4) symmetry. To implement this restoration and to fix the model parameters,
we resort to the energetics of Gamow-Teller resonances and to the minima of the
single $\beta^+$-decay strengths. This makes the theory predictive regarding
the $\beta\beta_{2\nu}$-decay, producing the $2\nu$ moments in $^{48}$Ca,
$^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{128,130}$Te, and $^{150}$Nd,
that are of the same order of magnitude as the experimental ones; however, the
agreement with $\beta\beta_{2\nu}$ data is only modest. To include
contributions coming from induced nuclear weak currents, we extend the
$\beta\beta_{0\nu}$-decay formalism employed previously in C. Barbero et. al,
Nuc. Phys. A628, 170 (1998). The numerical results for the $\beta\beta_{0\nu}$
moments in the above mentioned nuclei are similar to those obtained in other
theoretical studies although smaller on averag by $\sim 40\%$. We attribute
this difference basically to the one-QRPA-method, employed here for the first
time, instead of the currently used two-QRPA-method. The difference is
partially due to the way of carrying out the restoration of the spin-isospin
symmetry. It is hard to say which is the best way to make the restoration,
since the $\beta\beta_{0\nu}$ moments are not experimentally measurable. The
numerical uncertainties in the $\beta\beta$ moments, related with i) their
strong dependence on the residual interaction in the p-p channel when evaluated
within the QRPA, and ii) lack of proper knowledge of single-particle energies,
have been quantified. It is concluded that the partial restoration of the SU
(4) symmetry is crucial in the description of the $\beta\beta$-decays,
regardless of the nuclear model used.
