W UMa-type systems in globular clusters

W UMa systems can be found everywhere in the Galaxy. They can be used as a distance tracer. Therefore, W UMa systems are very important to investigate the structure of the Galaxy. The distance to W UMa systems in globular clusters (GCs) is determined using a period-color-luminosity relation. It is found that the mean distance (ra) of W UMa systems is consistent with their host cluster distances (rGC) deduced from their intrinsic distance moduli if rGC < 10 kpc. There is a significant difference between ra and rGC for rGC ^ 10 kpc. We discuss the reasons causing this deviation.