Bound states in dimerized and frustrated Heisenberg chains

Using the Bond-Operator Technique (BOT), we have studied the low energy exc itation spectrum of a frustrated dimerized antiferromagnetic Heisenberg cha in. In particular, we have compared our analytical results with previous Ex act Diagonalization (ED) data. Qualitatively, the BOT results are in good a greement with the ED data. And even a very good quantitative agreement is o btained in some parameter region. It is clearly shown that there is only on e elementary excitation branch (lowest triplet branch) and that the two oth er well defined excitations which appear below the continuum, one singlet a nd one triplet, are bound states of two elementary triplets.