The use of the (B-I) color index and applications of the (B-I) versus (B-V) relationship I. Open clusters.

We present an analysis derived from the UBVRI photometry of 12 open clusters, that shows a linear relationship between the color indices (B-I) and (B-V). Due to this characteristic the (B-I) color index is a more sensitive estimator of the effective stellar temperatures with respect to the widely used (B-V), and it is less affected by the absorption bands of H_2_ molecules in low mass stars. Furthermore, from the two colors (B-I) vs. (B-V) diagrams we derive a new method yielding a parameter that estimates the interstellar reddening for each cluster.