We study the sodium D lines ( D1 : 5895.92 Å ; D2 : 5889.95 Å ) in late-type dwarf stars .
The stars have spectral types between F6 and M5.5 ( B - V between 0.457 and 1.807 ) and metallicity between [ Fe / H ] = -0.82 and 0.6 .
We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO .
The observations have been performed periodically since 1999 .
The spectra were calibrated in wavelength and in flux .
A definition of the pseudo-continuum level is found for all our observations .
We also define a continuum level for calibration purposes .
The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index ( B - V ) of the stars .
When possible , we perform a careful comparison with previous studies .
Finally , we construct a spectral index ( R ^ { \prime } _ { D } ) as the ratio between the flux in the D lines , and the bolometric flux .
We find that , once corrected for the photospheric contribution , this index can be used as a chromospheric activity indicator in stars with a high level of activity .
Additionally , we find that combining some of our results , we obtain a method to calibrate in flux stars of unknown colour .