The Dependency of the Cepheid Period-Luminosity Relation on Chemical
  Composition
  The dependency of the Cepheid Period-Luminosity Relation on chemical
composition at different wavelengths is assessed via direct detailed abundance
analysis of Galactic and Magellanic Cepheids, as derived from high resolution,
high signal-to-noise spectra. Our measurements span one order of magnitude in
iron content and allow to rule out at the ~ 9 sigma level the universality of
the Period-Luminosity Relation in the V band, with metal rich stars being
fainter than metal poor ones by ~0.3 mag. The dependency is less pronounced in
the K band. Its magnitude and statistical significance decisively depend on
detailed distance measurements to individual stars, as inferred via the
Infrared Surface Brightness Method.
