It can be shown that cepheids pulsating in the first harmonic when evolving through the instability zone at constant bolometric luminosity and mass, are at a given period 0.09 magnitudes bluer than stars pulsating in the fundamental mode. Bell and Rodgers have shown that effective temperatures of cepheids can be obtained by comparison of observed and computed profiles of the wings of the Hα line. Hα cores of cepheids are affected by strongly doppler broadened chromospheric emission and absorption. Using a modified Oke effective temperature-intrinsic colour relation, observations of the Hα line of the Eggen C cepheids β Dor and S Mus have been made at Mt. Stromlo and intrinsic colours obtained for these stars.