BEAT CEPHEIDS AS GALACTIC METALLICITY TRACERS

We give a brief overview of Cepheids and of their modeling, with particular emphasis on F/O1 Beat Cepheids. Then we revisit the use of Period Ratio vs. Period diagram (Petersen diagram) for fundamental/first overtone Beat Cepheids, because they allow one to put very tight constraints on their metallicity Z. The Petersen diagram is shown to be largely independent of the helium content Y, of the mass-luminosity relation that is used in their construction, and of stellar rotation rates. However, it shows sensitivity to the chemical makeup of the elements that are lumped into the metallicity parameter Z. The Petersen diagram for the new Asplund, Grevesse & Sauval (2005) solar mix is compared to that for for the older, "standard" solar mix of Grevesse & Noels (1993).