The ESO All-Southern-Sky Redshift Survey of ROSAT Clusters of Galaxies

We discuss the rationale and the present status of a large redshift survey aiming at measuring distances for all the clusters of galaxies detected by the ROSAT X--ray All--Sky Survey in the southern hemisphere, with flux larger than $\sim 2\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$. The survey is being performed using three ESO telescopes in parallel. The final sample will contain about 700 clusters. It will represent a unique database for the study of the spatial distribution of clusters, their X--ray luminosity function and their intrinsic physical properties. Serendipitous results obtained so far include two new gravitational arc systems in two clusters of galaxies. These findings, obtained on rather short exposures, confirm the advantage of searching for arcs in X--ray selected clusters.