  We consider a special case of the n-component cubic model on the square
lattice, for which an expansion exists in Ising-like graphs. We construct a
transfer matrix and perform a finite-size-scaling analysis to determine the
critical points for several values of n. Furthermore we determine several
universal quantities, including three critical exponents. For n<2, these
results agree well with the theoretical predictions for the critical O(n)
branch. This model is also a special case of the ($N_\alpha,N_\beta$) model of
Domany and Riedel. It appears that the self-dual plane of the latter model
contains the exactly known critical points of the n=1 and 2 cubic models. For
this reason we have checked whether this is also the case for 1<n<2. However,
this possibility is excluded by our numerical results.
