The Metallicity dependence of the Cepheid Period-Luminosity relation:   methodology and results
  We present the results of an observational campaign undertaken to assess the influence of the iron content on the Cepheid Period-Luminosity relation. Our data indicate that this dependence is not well represented by a simple linear relation. Rather, the behaviour is markedly non monotonic, with the correction peaking at about solar metallicity and declining for higher and lower values of [Fe/H]. 
Marta Mottini (1), Martino Romaniello (1), Francesca Primas (1),   Martin Groenewegen (2), Giuseppe Bono (3), Patrick Franco...