  We study two components of the boundary of the compactification of the
variety I_3 of instantons of degree three. We use the desciption of I_3 as
symetric (involutive) cubo-cubic transforms deduced from the Beilinson monade.
It involves some geometry of curves and surfaces in P^3. This allows us to
distinguish two irreducible components which are in the closure of involutive
cubo-cubic transforms. It gives us two irreducible components of the boundary
of I_3. Moreover, we show that the cubo-cubic transforms of one of these
components are the inverse of the other one.
