Cepheids with Giant Companions: A New, Abundant Source of Cepheid Astrophysics

We present a progress report of our project aiming to increase the number ofknown Cepheids in double-lined binary (SB2) systems from six to 100 or more.This will allow us, among other goals, to accurately measure masses for a largesample of Cepheids. Currently, only six accurate Cepheid masses are available,which hinders our understanding of their physical properties and renders theCepheid mass–luminosity relation poorly constrained. At the same time,Cepheids are widely used for essential measurements (e.g., extragalacticdistances, the Hubble constant). To examine Cepheid period–luminosityrelations, we selected as binary candidates Cepheids that are too bright fortheir periods. To date, we have confirmed 56 SB2 systems, including thedetection of significant orbital motions of the components for 32. Weidentified systems with orbital periods up to five times shorter than theshortest reported period to date, as well as systems with mass ratiossignificantly different from unity (suggesting past merger events). Bothfeatures are essential to understand how multiplicity affects the formation anddestruction of Cepheid progenitors and what effect this has on global Cepheidproperties. We also present eight new systems composed of two Cepheids (onlyone such system was known before). Among confirmed SB2 Cepheids, there are alsoseveral wide-orbit systems. In the future, these may facilitate independentaccurate geometric distance measurements to the Large and Small MagellanicClouds.