Physical Realizations ofn>~4Vector Models

It is pointed out that certain phase transitions which involve a doubling of the unit cell are described by $n$-component vector models with $n>~4$. In particular, it is noted that the structural transition in Nb${\mathrm{O}}_2$ is described by an $n=4\mathrm{}$ component model with some tetragonal anisotropy. The critical behavior of this model is studied to order ${\epsilon }^2$, by the exact renormalization group in $d=4-\epsilon$ dimensions. It is found that the critical behavior is determined by a new, tetragonal, fixed point.