The population I Cepheids with 1.8 < P < 7 d are a non-homogeneous group in the sense that in this period range one can find fundamental, first overtone and double-mode pulsators. This implies that these three types of stars fall approximately in the same luminosity range. The knowledge of accurate physical parameters of these stars is therefore of paramount relevance in order to detect possible significant differences among different mode pulsators.

We have made spectrophotometric observations at La Silla Observatory of six Cepheids with P < 4 d (3 first-overtone and 3 fundamental mode pulsators). From a comparison with Kurucz's models of atmospheres (1979) we have derived their Teff's during the pulsational cycle. Due to the relevance of a correct estimate of the interstellar reddening in order to obtain accurate temperatures, we have independently determined the colour excesses of the studied objects by means of observations in the uvbyβ system using the method proposed by Feltz and McNamara (1980).