The system parameters of the ζ Aur-type binaries, all containing a cool supergiant and a hot dwarf, have been improved recently by IUE observations (N.P. Schroeder, 1985; A.Ap.147, 103). But the diameters of the supergiant components, determined with different indirect methods, are still discordant. We attempted to solve such discrepancies by measuring directly the angular sizes of these stars, expected to be of a few milli-arcseconds.