Null vectors of the W_3 algebra
  We construct $W_3$ null vectors of a restricted class explicitly in two different forms. The method we use is an extension of that of Bauer et al.~in the Virasoro case. Our results are analogous to the formulae of Benoit and St.~Aubin for the Virasoro null vectors. We derive in the Virasoro case some alternative formulae for the same null vectors involving only the $L_{-1}$ and $L_{-2}$ modes of the Virasoro algebra. } 
P. Bowcock and G.M.T. Watts