Infrared photometry of Magellanic Cloud Cepheids. Intrinsic properties of Cepheids and the spatial structure of the Clouds

JHK photometry has been obtained for 21 Cepheids in the LMC and 23 Cepheids in the SMC. From dereddened mean magnitudes and colours, the period-luminosity, period–colour and period–luminosity–colour relations in the infrared are derived. The colour term in the infrared is found to be significant, not caused by differential reddening, and in agreement with the theoretical value. Both the structure of the LMC and the SMC can be approximated by inclined planes, but the structure in the SMC is more complex and shows much greater depth effects than in the LMC. The Cepheids in the wing of the SMC are closer than the main body and there is a group of Cepheids at the southwestern end of the bar which are 0.34 mag more distant. The correlation of the structure of the SMC with the H I radio maps is discussed. With the zero point of the infrared PL and PLC relations determined by infrared observations of galactic calibrating Cepheids in the same period range, the distance moduli to the Clouds are $$(m-M)_0=18.73\enspace (\text {LMC})$$ and $$(m-M)_0=19.05$$ (SMC main body), corresponding to distances of 56 and 65 kpc respectively.