  Let k be a perfect field, and K be a totally ramified extension of K_0 = Frac
W(k) of degree e. To a semi-stable p-adic representation of G_K (the absolute
Galois group of K), one can classicaly associate two polygons : the Hodge
polygon et the Newton polygon. It is well known that the former lies below the
latter, and that they have same endpoints. In this note, we introduce a third
polygon gotten from the semi-simplification of the representation mod p, and,
under some conditions on Hodge-Tate weights, we prove that it lies above the
Hodge polygon again with same endpoint. We finally examine one exemple
associated to a crystalline representation.
