diff --git a/src/SystemIdentification.jl b/src/SystemIdentification.jl
index 364e1ed7e4cee4d3ac12a4a39dbf4a8b5a2ebbbd..655bec8d051137b86304a597a0e7531fab5dcc19 100644
--- a/src/SystemIdentification.jl
+++ b/src/SystemIdentification.jl
@@ -4,6 +4,7 @@ export
 Model,LinearModel,NonLinearModel,
 Network,
 Polynom,PolynomMatrix,
+TF,
 AR,
 ARX,
 RBFARX,
@@ -58,6 +59,11 @@ type ARX <: LinearModel
     nb::Polynom{Int}
 end
 
+type TF <: LinearModel
+    P
+    F
+end
+
 type RBFARX <: Network
     na::Int
     n_centers::Int