Exceptional coupling constants for the Coulomb-Dirac operator with anomalous magnetic moment

It was recently shown that the point spectrum of the separated Coulomb-Dirac operator H_0(k) is the limit of the point spectrum of the Dirac operator with anomalous magnetic moment H_a(k) as the anomaly parameter tends to 0; this spectral stability holds for all Coulomb coupling constants c for which H_0(k) has a distinguished self-adjoint extension if the angular momentum quantum number k is negative, but for positive k there are certain exceptional values for c. Here we obtain an explicit formula for these exceptional values. In particular, it implies spectral stability for the three-dimensional Coulomb-Dirac operator if |c| < 1, covering all physically relevant cases.