  Pure CFTs have vanishing $\beta$-function at any value of the coupling. One
example of a pure CFT is the O(N) Wess-Zumino model in 2+1 dimensions in the
large N limit. This model can be analytically solved at finite temperature for
any value of the coupling, and we find that its entropy density at strong
coupling is exactly equal to 31/35 of the non-interacting Stefan-Boltzmann
result. We show that a large class of theories with equal numbers of
N-component fermions and bosons, supersymmetric or not, for a large class of
interactions, exhibit the same universal ratio. For unequal numbers of fermions
and bosons we find that the strong-weak thermodynamic ratio is bounded to lie
in between 4/5 and 1.
