Problems and possibilities in fine-tuning of the Cepheid P-L   relationship
  Factors contributing to the scatter around the ridge-line period-luminosity relationship are listed, followed by a discussion how to eliminate the adverse effects of these factors (mode of pulsation, crossing number, temperature range, reddening, binarity, metallicity, non-linearity of the relationship, blending), in order to reduce the dispersion of the P-L relationship. 
L. Szabados and P. Klagyivik
astro-ph.SR