The frequency of Cepheids with companions: a photoelectric approach

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Abstract
The effects of main sequence companions on the colour curves of Cepheids are investigated. It is shown that the degree of opening of the loop and the shape of the trajectory of a Cepheid, as a function of phase, in the (U–B)/(B–V) diagram are both strong functions of the spectral type of the companion and the magnitude difference between the companion and the Cepheid. The colour–colour plots of 185 Cepheids with UBV photoelectric photometry have been inspected: 38 Cepheids have well-opened loops and an additional 13 show positive signs of opening. Interpreted strictly in terms of companions these statistics imply that at least 20 per cent of all Cepheids have companions with an additional 7 per cent as probable candidates. The method fails to detect companions more than 4 mag fainter than the Cepheid.

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