[en] For improved measurements of the key astrophysical reaction ¹²C(α, γ)¹⁶O in inverse kinematics, a recoil separator ERNA is developed to detect directly the ¹⁶O recoils with nearly 100% efficiency. Since the ¹²C projectiles and the ¹⁶O recoils have essentially the same momentum and since the ¹²C ion beam emerging from an accelerator usually passes through a momentum filter, a sufficient absence of an ¹⁶O beam contaminant in the ¹²C ion beam is of utmost importance for ERNA. In the present work, a Wien filter together with a ΔE-E telescope are used to investigate the beam contaminants accompanying a momentum-filtered ¹²C ion beam and to measure the level of ion beam purification achievable with the Wien filter