  We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) 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_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.
