2002A&A...385.1107K

We present a number of relationships between similar colour indices in the Vilnius and Stroemgren photometric systems, empirically derived from all observational data available. Significant differences between the corresponding indices, apparently due to the different central wavelength and width of the Stroemgren u and v bands and Vilnius U and X bands are found. We demonstrate that the effects may depend on metallicity and luminosity. For G-K stars of moderate luminosity we interpret the difference introduced by v and X pass-bands as an log g effect, reflecting the presence of CaII lines in these spectral types. Apart from the interstellar CaII absorption, the difference (v-X) is little affected by the interstellar dust. It turns out as well to be very straightforward to make an estimation of the [Fe/H] index with an accuracy of 0.4 dex, using the Vilnius U and Stroemgren u filters. A comparison of (v-X) to the hk index is also presented, supporting the conclusion that X magnitude is strongly influenced by the CaII lines. We discuss a possible influence of the CaII stellar lines on the absolute magnitude inferred from the Vilnius photometry.