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The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of Cepheids and the Effect of Metallicity on the CO Band-head at $4.6μ$m

We compare mid-infrared 3.6 and 4.5 $μ$m Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the CO rotation-vibration band-head at 4.6 $μ$m on the mid-infrared photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 $μ$m and find no evidence for an effect at 3.6 $μ$m. We compare the ([3.6]-[4.5]) period-colour relations in the MW, LMC, and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al. (2014), demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.