Ethylene copolymers with octene and/or ethylene terpolymers with alpha-olefins from 3 to 12 carbon atoms, optionally in the presence of other comonomers containing more than one unsaturation, usable as additives to increase the gas oil properties at low temperatures, obtainable by polymerization of the monomers in the presence of catalysts comprising: a bis-cyclopentadienyl derivative having the general formula: (Cp.sub.1 Cp.sub.2)--M--(L.sub.2 L.sub.3) containing groups with oxygen bound to the transition metal, wherein M is a metal from the IIIb group to the Vb group or of the lanthanide series of the Element Periodic Table; Cp.sub.1 and Cp.sub.2, equal to or different from each other, represent cyclopentadienyls bound to M with delocalized .pi. bonds.