a discussion on metallicity effects on the period-luminosity relations of cepheids and miras

the period-luminosity relations (hereafter plrs) of classical cepheids and miras are important distance indicators, but the unsolved metallicity effects remain as a critical source of uncertainty. the plrs in the near infrared (ir) bands have become important due to the increasing evidence for significantly smaller metallicity effects than in the optical bands. our recent near-ir survey revealed classical cepheids, in addition to previously known miras, in the galactic centre. on the other hand, these types of variables in the magellanic clouds have been well searched for in the optical gravitational lensing experiment (ogle), and there exist near-ir catalogues which include those objects. these allow us a direct comparison of the distance indicators found in different environments. here we compare the near-ir plrs of classical cepheids and oxygen-rich miras in the galactic centre and the large and small magellanic cloud. in this way, we can discuss the metallicity effects on their plrs, in a relative sense, without the knowledge of the true distances to the stellar systems. we found that the metallicity effect on the logp–ks relation of cepheids, if any, is similar to the case of miras, to the limit of 0.1 mag when we compare these objects in the magellanic clouds.