Introduction.

As suggested in ref. [#1##1###] the presence of a large scalar potential in a relativistic version of the nucleon selfenergy in the nucleus [#2##1###] leads to a sizeable renormalization of the axial charge in nuclei. This renormalization, which is also sometimes refered to as the heavy meson exchange current contribution, must be considered in addition to the conventional meson exchange currents studied earlier [#3##1###,#4##1###,#5##1###,#6##1###]. More quantitative evaluations of this renormalization, following the idea of [#1##1###], have been recently provided in [#7##1###,#7a##1###,#8##1###]. In ref. [#7##1###] a perturbative approach is used starting from a relativistic description of the NN potential and taking direct and exchange terms. The strong short-range and tensor components of a realistic NN interaction give rize to significant two-nucleon correlations. The effects of NN correlations are taken into account in the investigations of ref. [#7a##1###] by using the Brueckner G-matrix. The estimates reported in [#7##1###] and [#7a##1###] were made for the system of infinite nuclear matter.

The investigations of ref. [#8##1###] are performed directly for finite nuclei. Also in this case the effect of the nucleon selfenergy is treated in a perturbative way. The operators are reduced to a bispinor representation and the calculations are carried out in a nonrelativistic frame. The single-particle wavefunctions are represented by oscillator wavefunctions and the effect of correlations are included in terms of a simple local correlation function.

In the present work we want to consider the relativistic features, the effects of correlations and the single-particle wavefunctions consistently. For that purpose we employ the results of the relativistic Dirac Brueckner Hartree Fock (DBHF) calculations of ref. [#10##1###]. These calculations are based on the version A of the relativistic One-Boson-Exchange potential of [#rupr##1###]. The results of the calculation of the ground-state properties of double closed-shell nuclei are in good agreement with the experimental data and the resulting self-energy yields a real part for the optical potential of low-energy nucleon nucleus scattering, which is close to the empirical analysis [#klein##1###].

After this short introduction we will review the perturbative treatment of the heavy meson exchange current contribution in nuclear matter. The self-consistent DBHF calculations are discussed in section 3, while section 4 contains a discussion of the non-relativistic reduction. The results are presented and discussed in section 5 and the final section summarizes the main conclusions.