The idea of Fernie (1980) to detect the presence of companions to cepheids from the phase-shift between the light and colour curves has been applied to the xtensive set of observations in the VBLUW photometric system published by Pel (1976)°. The distribution of Delta Phi values shows that approximately the 25% of the cepheids considered have possible companions. Analyzing then a series of simulated binary cepheids, we find that the Walraven photometry does not seem very suitable to calibrate the phase-shift in terms of the difference of magnitude between the cepheid and the blue companion. However, the simulation has been extended to the study of the effect of (unresolved) companions on the cepheid mean radius computed by means of the C0RS method of Caccin et al. (1980, 1981), showing the stability of this method also with this source of uncertainity.
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