THE USE OF THE (B-I) COLOR INDEX AND APPLICATIONS OF THE (B-I) VERSUS(B-V) RELATIONSHIP .1. OPEN CLUSTERS

We present an analysis derived from the UBV RI photometry of 12 open c lusters, 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 wi th 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 yi elding a parameter that estimates the interstellar reddening for each cluster.