RR Lyrae's have long been recognized to be excellent tracers of old stellar populations, as well as primary distance indicators for Population II systems. The identification of RR Lyrae stars in composite systems, such as Local Group galaxies, is unambiguous signature of the presence of old, metal-poor stars in these systems. By comparing the properties of RR Lyrae in nearby galaxies with those of the Milky Way variables, the metallicity and age of the oldest population can be estimated.
Being the primary distance indicators in the Galaxy and in the Magellanic Clouds, RR Lyrae stars are corner-stones of cosmological distance and time scales. For instance, the distance to the LMC from Population II objects is finally founded on the luminosity of the RR Lyrae variables.
The absolute magnitude of the RR Lyrae variables is known to depend on metallicity, but no general consensus has been reached so far both on the correct slope and on the zero-point of the Mv(RR)-[Fe/H] relation. In turn, a dichotomy at a 0.2-0.3 mag level is found to exist between the short distance provided by the Baade-Wesselink and Statistical parallax methods applied to field RR Lyraes, and the long distance to the LMC as derived from the Main Sequence Fitting of the Globular Clusters and the Population I distance indicators, Cepheids in particular.
Since a very fine tuning of the physical parameters (mass and mass-loss in particular) is required to lead a star evolving along the Red Giant Branch to fall inside the instability strip of the Horizontal Branch, RR Lyraes represent an invaluable test of stellar evolution and pulsation theories. The presence of scatter and large amplitude variations in the light curves are often the signature of double-mode pulsation and Blazhko effect in some of these variables. These phenomena, and the double-mode pulsation in particular, offer a unique opportunity to derive direct informations on the stellar parameters (mass, mass-metallicity relation), structure (magnetic fields, etc.), pulsation mechanism (resonance, mode mixing), as well as on direction and rate of evolution across the Horizontal Branch.
A large number of research programs at various levels of completion are presently carried out, and new projects are proposed to address the RR Lyrae distance scale and the stellar population issues, as well as the impact of the RR Lyraes on stellar evolution and pulsation theories (see sections 1.4.1, 1.4.2, 1.4.3, and 1.4.4 below).
New software tools are being developed, purposely designed to allow the identification of variable stars and to perform period searches and studies of multi periodicities of variable objects.
An effort is also being made to organize the observational material on RR Lyrae variables already in hand or coming from future scheduled observations, in order to made it available in electronic form to the astronomical community (see section 1.4.5).