  We present a progress report of our project aiming to increase the number of
known 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 large
sample of Cepheids. Currently, only six accurate Cepheid masses are available,
which hinders our understanding of their physical properties and renders the
Cepheid mass--luminosity relation poorly constrained. At the same time,
Cepheids are widely used for essential measurements (e.g., extragalactic
distances, the Hubble constant). To examine Cepheid period--luminosity
relations, we selected as binary candidates Cepheids that are too bright for
their periods. To date, we have confirmed 56 SB2 systems, including the
detection of significant orbital motions of the components for 32. We
identified systems with orbital periods up to five times shorter than the
shortest reported period to date, as well as systems with mass ratios
significantly different from unity (suggesting past merger events). Both
features are essential to understand how multiplicity affects the formation and
destruction of Cepheid progenitors and what effect this has on global Cepheid
properties. We also present eight new systems composed of two Cepheids (only
one such system was known before). Among confirmed SB2 Cepheids, there are also
several wide-orbit systems. In the future, these may facilitate independent
accurate geometric distance measurements to the Large and Small Magellanic
Clouds.
