  The isotope shifts of forbidden transitions in Be- and B-like argon ions are
calculated. It is shown that only using the relativistic recoil operator can
provide a proper evaluation of the mass isotope shift, which strongly dominates
over the field isotope shift for the ions under consideration. Comparing the
isotope shifts calculated with the current experimental uncertainties indicates
very good perspectives for a first test of the relativistic theory of the
recoil effect in middle-Z ions.
