A class of metallocenes of formula (I): (ZR1m)n(Cp)(A)rMLpL′q(I) wherein (ZR1m)n, is a divalent group bridging Cp and A, Cp is a heterocyclic cyclopentadienyl group of formula (II) or (II′), wherein one of X or Y is a single bond, the other being an heteroatom; R2 and R3 are hydrogen, halogen or a hydrocarbon radical, optionally containing heteroatoms; R4 is halogen or an hydrocarbon radical, optionally containing heteroatoms; the group A is a cyclopentadienyl derivative or is equal to Cp; M is Ti, Zr or Hf; m is 1 or 2; n ranges from 0 to 4; r is 0 or 1; p and q range from 0 to 3; and a ranges from 0 to 4. Moreover, are disclosed catalyst systems containing these metallocenes, useful in the polymerization of olefins.