The Carnegie Chicago Hubble Program: The Mid-Infrared Colours of
  Cepheids and the Effect of Metallicity on the CO Band-head at $4.6\mu$m
  We compare mid-infrared 3.6 and 4.5 $\mu$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 $\mu$m on the mid-infrared photometry. We confirm the temperature
sensitivity of the CO band-head at 4.6 $\mu$m and find no evidence for an
effect at 3.6 $\mu$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.
