  We present an observational program we started in 1999, to systematically
obtain mid-resolution spectra of late-type stars, to study in particular
chromospheric activity. In particular, we found cyclic activity in four dM
stars, including Prox-Cen. We directly derived the conversion factor that
translates the known S index to flux in the Ca II cores, and extend its
calibration to a wider spectral range. We investigated the relation between the
activity measurements in the calcium and hydrogen lines, and found that the
usual correlation observed is the product of the dependence of each flux on
stellar color, and it is not always preserved when simultaneous observations of
a particular star are considered. We also used our observations to model the
chromospheres of stars of different spectral types and activity levels, and
found that the integrated chromospheric radiative losses, normalized to the
surface luminosity, show a unique trend for G and K dwarfs when plotted against
the S index.
