We introduce a vector model coupled to an N-colour Ashkin-Teller model and
solve its phase diagram in the large-N limit in two dimensions. It is shown
that the transition line starts from the axis of Ising couplings with a be
haviour which can be critical or first order depending on the strength of t
he four-spin coupling and of the coupling between spin and vector variables
. Below a negative value of the four-spin coupling the transition is always
continuous.