$W_3$ string scattering
  The group theoretic method is extended to include fields with a background charge. This formalism is used to compute the tree level scattering for $W_3$ strings. The scattering amplitudes involve Ising model correlation functions. A detailed study of the four tachyon amplitude shows that the $W_3$ string must possess additional states in its spectrum associated with intercept $1/2$ and the energy operator of the Ising model. 
M. D. Freeman and P. West