Blue companions of Cepheids

Twenty-one Cepheids, known or suspected to have blue companions, were studied with the International Ultraviolet Explorer satellite. For 13 of them, companions were indeed seen, though they were generally fainter in the UV than expected. For four Population I Cepheids, the suspected companions were not seen. For none of the Population II Cepheids could a companion be detected. The effective temperatures and luminosities of the companions which could be observed are discussed, and the positions of Cepheids and companions in the T(eff) luminosity diagrams are compared with positions expected from stellar evolution calculations.