Eclatement de r\'eseaux de quadriques et bord des instantons de degr\'e 3

In their article [1], L. Gruson and M. Skiti have constructed a birationnal map from the variety $\I$ of mathematical instantons of degree 3 to the variety of nets of quadrics in $\pd$. They describe by this way two irreducible componants of the boundary of $\I$ associated to the divisor of nets which contain a two-plane degenerated quadric and the divisor of L\" uroth nets. In this article we describe an irreducible componante of the boundary of $\I$ as the exceptionnal divisor of the blowing-up of the closed set of nets of quadrics of rank 3.