Top panel: evolution of the Doppler velocity at maximum absorption of the Fe iiλ5169 feature for 32 SE SNe. SNe IIb, Ib, Ic, and Ic-BL are represented in green, blue, red, and magenta, respectively. The SNe IIb, Ib, and Ic follow a similar evolution that can be represented by a PL function. The SNe Ic-BL are found to exhibit higher velocities at all epochs and have therefore been excluded from the PL fit (dashed line). Middle panel: velocity evolution of He iλ5876 for SNe IIb (green) and Ib (blue), fitted by a PL. Bottom panel: velocity evolution of O iλ7772 for SNe Ic (red) and Ic-BL (magenta), fitted by a PL.