  In a previous paper we found that the isospin susceptibility of the O($n$)
sigma-model calculated in the standard rotator approximation differs from the
next-to-next to leading order chiral perturbation theory result in terms
vanishing like $1/\ell\,,$ for $\ell=L_t/L\to\infty$ and further showed that
this deviation could be described by a correction to the rotator spectrum
proportional to the square of the quadratic Casimir invariant. Here we confront
this expectation with analytic nonperturbative results on the spectrum in 2
dimensions, by Balog and Heged\"us for $n=3,4$ and by Gromov, Kazakov and
Vieira for $n=4$. We also consider the case of 3 dimensions.
