RS Gruis: a dwarf cepheid in a binary system

Abstract
Simultaneous BVRI photometry and photoelectric radial velocities are presented for the dwarf cepheid (AI Vel star) RS Gru. It is shown that the centre of mass velocity varies from cycle to cycle and clearly indicates the presence of a companion. The data are insufficient to determine the orbital period. A modified Wesselink analysis is used to determine a radius of 2.9 solar radii. This is consistent with an immediate post-main sequence state of evolution. The Barnes–Evans method of distance estimation is not applicable because of the presence of the companion.

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