Discovery of blue companions to two southern Cepheids: WW Car and FN Vel

TL;DR

This study introduces a spectroscopic method to detect blue companions to Cepheid stars by analyzing the CaII H+K spectral lines, successfully identifying new companions to WW Car and FN Vel, even when orbital velocities are undetectable.

Contribution

The paper presents a novel spectroscopic technique for identifying blue companions to Cepheids, effective regardless of orbital velocity variations.

Findings

Confirmed blue companions to WW Car and FN Vel

Method effective for systems with undetectable orbital velocities

Blue companions identified as hot stars

Abstract

A large number of high-dispersion spectra of classical Cepheids were obtained in the region of the CaII H+K spectral lines. The analysis of these spectra allowed us to detect the presence of a strong Balmer line, H$\epsilon$, for several Cepheids, interpreted as the signature of a blue companion: the presence of a sufficiently bright blue companion to the Cepheid results in a discernible strengthening of the CaII H + Hepsilon line relative to the CaII K line. We investigated 103 Cepheids, including those with known hot companions (B5-B6 main-sequence stars) in order to test the method. We could confirm the presence of a companion to WW Car and FN Vel (the existence of the former was only suspected before) and we found that these companions are blue hot stars. The method remains efficient when the orbital velocity changes in a binary system cannot be revealed and other methods of binarity detection are not efficient.