  We study the generalised constrained BF theory described in gr-qc/0102073 in
order to introduce the Immirzi parameter in spin foam models. We show that the
resulting spin foam model is still based on simple representations and that the
generalised BF action is simply a deformation of the Barrett-Crane model. The
Immirzi parameter doesn't change the representations used in the spin foam
model, so it doesn't affect the geometry of the model. However we show how it
may still appear as a factor in the area spectrum.
