Abstract
The rate of binaries among cepheid stars is 25 - 35% (Burki, 1984a), a value which is in agreement with the rate among F-M supergiants (31-38%) obtained by Burki and Mayor (1983) from a radial velocity survey.For binary cepheids, the Wesselink radius RW is affected by the light of the companion. Balona (1977) found that RW is too small if the companion is a blue main sequence star and too large if it is a red giant star. This effect has been quantitatively examined by Burki (1984a). For example, it was found that, in the case of a cepheid of period P = 32d with a companion fainter by 2 mag., RW is 70% of its correct value if we are dealing with a main sequence companion and 130% in the case of a companion redder by 0.6 in B-V. The bias on RW becomes negligible when the companion is fainter than the cepheid by more than 4 mag.
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