The existence of a factorized unbounded operator between Frechet spaces

For locally convex spaces E and F, the continuous linear map T : E -> F is called bounded if there is a zero neighborhood U of E such that T(U) is bounded in F. Our main result is that the existence of an unbounded operator T between Frechet spaces E and F which factors through a third Frechet space G ends up with the fact that the triple (E, G, F) has an infinite dimensional closed common nuclear Kothe subspace, provided that F has the property (y).