A HgMn companion to the Cepheid SU Cyg.

Abstract

We report here the discovery of a chemically peculiar star of the HgMn type as a companion to the classical Cepheid variable SU Cyg. This is the first investigation of a HgMn star in a binary system in which a star of a more advanced evolutionary stage is the primary. The age of the Cepheid provides an estimate to the age of the chemical peculiarities which is independent of the photometric calibration of HgMn field stars. From a synthetic spectrum analysis it is determined that the HgMn star, SU Cyg B, displays the abundance deficiency for nitrogen and enhancements for manganese (+1.5\,dex) and the very heavy elements mercury (+5.8\,dex) and platinum (+5.0\,dex). Absorption from the strong transitions Au Ii lambda1740 and Tl Ii lambda1908 each implies abundance enhancements of 3.0\,dex. A search through the IUE and HST/GHRS spectral archives reveals several binary-Cepheid companions which are candidates for chemically peculiar stars.