Computation of the distance moduli of RR Lyrae stars from their light and colour curves

We use B and V data of globular cluster variables to derive a formula for the distance moduli of RRab stars. The method employs the Fourier decomposition of the V light curve and the averageB−V colour index. By using our former result for the V0 absolute magnitude, we also obtain an expression for the dereddened colour index. With the aid of the new formulae, the relative distance moduli can be estimated within an error of < 0.03 mag. Although we also make an absolute calibration, it is cautioned that this may be more affected by possible systematic errors originating mostly from the Baade−Wesselink magnitudes. On the basis of the scatter of the individual distance moduli computed with and without reddening correction, it is shown that inhomogeneous reddening plays a role in several clusters. By using our formulae we derive new expressions for the Ic and K absolute magnitudes on a sample of stars which contains mostly field stars with accurate photometry. As a byproduct of this derivation we also give optimum estimations for the selective absorption coefficient RV . We show that the K absolute magnitude contains important contribution also from the Fourier parameters, besides the well known dependence on the period. The Ic absolute magnitude is superbly correlated with the Fourier parameters, which implies that this colour is a very good candidate for the accurate estimation of the absolute magnitude.