The spectral energy distributions of very long-period Cepheids in the Milky Way, the Magellanic Clouds, M31, and M33

The spectral energy distributions (SEDs) of 20 Milky Way (MW), 9 Large Magellanic Cloud (LMC), 7 Small Magellanic Cloud (SMC), 12 M31, and 7 M33 (classical) Cepheids with periods longer than 50 days were constructed using photometric data from the literature and fitted with model atmospheres with the aim of identifying objects with an infrared excess. The SEDs were fitted with stellar photosphere models to derive the best-fitting luminosity and effective temperature; a dust component was added when required. The distance and reddening values were taken from the literature. WISE and IRC images were inspected to verify whether potential excess emission was related to the central objects. Only one star with a significant infrared (IR) excess was found in the LMC and none in the SMC, M31, and M33, contrary to earlier work on the MW suggesting that IR excess may be more prominent in MW Cepheids than in the Magellanic Clouds. One additional object in the MW was found to have an IR excess, but it is unclear whether it is a classical Cepheid or a type-II Cepheid. The stars were plotted in a Hertzsprung-Russell diagram (HRD) and compared to evolutionary tracks for CCs and to theoretical instability strips. For the large majority of stars, the position in the HRD is consistent with the instability strip. For stars in the MW uncertainties in the distance and reddening can significantly change their position in the HRD.