Chromospheric Long Term Activity of Very Cool Stars

We study the long-term chromospheric activity of dM stars, using in particular the results of an observational program we started in 1999, to systematically obtain mid-resolution spectra of late-type stars. First, we discuss which are good activity indicators for these stars. We investigated the relation between the activity measurements in the calcium and hydrogen lines, and found that the usual correlation observed reflects differences between stars, and it is not always preserved for simultaneous observations of a particular star. Later we investigate the variations in individual stars, and we report cyclic activity in four dM stars, including Prox-Cen.