We use a large sample of RRab stars in globular clusters and in the Sculptor dwarf galaxy to decipher the relation between the Fourier decomposition and the luminosity .
For fixing the zero point of the relation , we use the latest Baade-Wesselink ( BW ) results .
It is shown that the most plausible representation of the absolute brightness ( in V color ) consists of a linear expression of the period and of the Fourier parameters A _ { 1 } and \varphi _ { 31 } ( computed also from the V light curve ) .
We derive an average M _ { V } = \alpha { [ Fe / H ] } + \beta relation from all the available Fourier data .
Our results exclude any values of \alpha larger than 0.19 , in agreement with most of the BW and evolutionary studies .
We give age and reddened distance modulus estimations for the clusters entering in our analysis .