Wilson & Bappu ( 1957 ) have shown the existence of a remarkable correlation between the width of the emission in the core of the K line of Ca { II } and the absolute visual magnitude of late–type stars .

Here we present a new calibration of the Wilson–Bappu effect based on a sample of 119 nearby stars .
We use , for the first time , width measurements based on high resolution and high signal to noise ratio CCD spectra and absolute visual magnitudes from the Hipparcos database .

Our primary goal is to investigate the possibility of using the Wilson–Bappu effect to determine accurate distances to single stars and groups .

The result of our calibration fitting of the Wilson–Bappu relationship is M _ { V } = 33.2 - 18.0 \cdot \log W _ { 0 } , and the determination seems free of systematic effects .
The root mean square error of the fitting is 0.6 magnitudes .
This error is mostly accounted for by measurement errors and intrinsic variability of W _ { 0 } , but in addition a possible dependence on the metallicity is found , which becomes clearly noticeable for metallicities below [ Fe/H ] \sim -0.4 .
This detection is possible because in our sample [ Fe/H ] ranges from -1.5 to 0.4 .

The Wilson–Bappu effect can be used confidently for all metallicities not lower than \sim -0.4 , including the LMC .
While it does not provide accurate distances to single stars , it is a useful tool to determine accurate distances to clusters and aggregates , where a sufficient number of stars can be observed .

We apply the Wilson–Bappu effect to published data of the open cluster M67 ; the retrieved distance modulus is of 9.65 magnitude , in very good agreement with the best distance estimations for this cluster , based on main sequence fitting .