Composite Mv versus (V-I)0 diagram for template open clusters

New CCD VI photometric data for 10 template open clusters with accurately determined fundamental parameters are presented. From the observed V versus V-I diagrams of the clusters, fiducial sequences have been defined and transformed into the M-v versus (V-I), diagram by averaging sequences of template open clusters with similar ages. The resulting composite M-v versus (V-I)(O) diagram presents a homogeneous set of empirical isochrones in the age range between 5 Myr and 4 Gyr. These empirical isochrones show an overall very good agreement with those computed from stellar evolutionary models. Theoretical isochrones with moderate overshooting fit the observed main sequences better than the canonical ones for clusters older than 600 Myr. The present set of empirical isochrones will be useful for the study of faint reddened open clusters.