The spectroscopic binary system GI 375 -: I.: Orbital parameters and chromospheric activity

Aims. We study the spectroscopic binary system G1 375 to characterise its orbit and the spectral types and chromospheric activity levels of the components. Methods. We employed medium-resolution echelle spectra obtained at the 2.15 m telescope at the Argentinian observatory CASLEO and photometric observations obtained from the ASAS database. Results. We have separated the composite spectra into those corresponding to both components. The separated spectra allow us to confirm that the spectral types of both components are similar ( dMe3.5) and to obtain precise measurements of the orbital period ( P = 1.87844 days), minimum masses ( M-1 sin(3) i = 0.35 M-circle dot and M-2 sin(3) i = 0.33 M-circle dot), and other orbital parameters. The photometric observations exhibit a sinusoidal variation with the same period as the orbital period. We interpreted this as signs of active regions carried along with rotation in a tidally synchronised system, and studied the evolution of the amplitude of the modulation on longer timescales. Together with the mean magnitude, the modulation exhibits a roughly cyclic variation with a period of around 800 days. This periodicity is also found in the flux of the Ca II K lines of both components, which seem to be in phase. Conclusions. The periodic changes in the three observables are interpreted as a sign of a stellar activity cycle. Both components appear to be in phase, which implies that they are magnetically connected. The measured cycle of approximate to 2.2 years (approximate to 800 days) is consistent with previous determinations of activity cycles in similar stars.