[en] The aim of our present work is to explore the role of the nuclear superhyperfine interactions with respect to this problem, and to see whether the fraction of nuclei having a non-zero spin may be considered responsible for such a behaviour by broadening the linewidths of the μ⁺SR signals. In undertaking this programme some valuable information can be drawn from old ESR and ENDOR measurements on shallow donors in Si and Ge. In fact the analogy with this kind of impurities, though not conclusive, has been so far the most useful in the understanding of the Musup(*) centre. (orig.)