“…In complete analogy with the spin-2 case we have shown that starting with the first-order non-gauge invariant self-dual model [7] it is possible to obtain the second- [8], third- [9] and fourthorder self-dual models, where the last one has all the auxiliary fields needed to correctly describe only one helicity +3 or −3 particle. In [6] we have faced the problem of missing gauge symmetries which are required in order to proceed with the technique and go beyond the fourth-order self-dual model, which might be naively expected since looking at the spin-1 and spin-2 examples, one can see that there are two and four self-dual descriptions for the singlets, respectively, indicating that there might be some 2s rule for the highest order of the spin-s self-dual model, where s is the spin. In fact the authors in [10] have proposed fifth-and sixth-order equations of motion, for a massive spin-3 particle, however, without considering auxiliary fields, where, however, the fifth-order model contains ghosts.…”