  We study the appearance of bound states in the spin gap of spin-1/2 ladders
induced by weak bond disorder. Starting from the strong-coupling limit, i.e.,
the limit of weakly coupled dimers, we perform a projection on the
single-triplet subspace and derive the position of bound states for the single
impurity problem of one modified coupling as well as for small impurity
clusters. The case of a finite concentration of impurities is treated with the
coherent-potential approximation in the strong-coupling limit and compared with
numerical results. Furthermore, we analyze the details in the structure of the
density of states and relate their origin to the influence of impurity
clusters.
