  Various properties of two kinds of massless representations of the
n-conformal (or (n+1)-De Sitter) group $\tilde{G}_n=\widetilde{SO}_0(2,n)$ are
investigated for $n\ge2$. It is found that, for space-time dimensions $n\ge3$,
the situation is quite similar to the one of the n=4 case for $S_n$-massless
representations of the n-De Sitter group $\widetilde{SO}_0(2,n-1)$. These
representations are the restrictions of the singletons of $\tilde{G}_n$. The
main difference is that they are not contained in the tensor product of two
UIRs with the same sign of energy when n>4, whereas it is the case for another
kind of massless representation. Finally some examples of Gupta-Bleuler
triplets are given for arbitrary spin and $n\ge3$.
