The impact of binary Cepheids on the distance determinations

Due to the light of their companions, binary Cepheids might seem over-luminous and appear above the period-luminosity relation (PLR). This systematic effect introduces a shift in the PLR zero-point towards smaller magnitudes. We present quantitative results on the impact that companions to classical Cepheids have on the PLR zero-point. We use the binary population synthesis code StarTrack to evolve 200,000 binary systems for three metallicities $Z$=0.004, 0.008, 0.02, which correspond respectively to the metallicities of the Small and Large Magellanic Clouds and the Milky Way. We narrow these populations down to stars which pass the filtering criteria for classical Cepheids on their second and third crossing through the instability strip, and parametrize the impact of companions in form of the binarity fraction $f_{\rm bin} = 25, 50, 75, 100$. This yields a linear relation between the binarity fraction and the shift of the PLR zero-point, and find that it is steeper at shorter wavelengths.