In the transmitted beam of a sodium cell with a single feedback mirror highly modulated patterns appear. The patterns possess the dihedral symmetries [ital D][sub 6], [ital D][sub 2], and [ital D][sub 3]. More complex patterns consisting of more than one elementary cell show a local hexagonal arrangement. The topological characteristics are very similar to the one of boundary affected patterns predicted in a thin Kerr slice. The instability is due to the competition of diffraction, diffusion, and optical pumping which is influenced by a weak magnetic field. For higher cell-to-mirror distances the observation of an additional length scale indicates a competition between the feedback and a further instability mechanism, probably the counterpropagating beam instability.