2007A&A...474..345D

We study the spectroscopic binary system Gl 375 to characterise its orbit and the spectral types and chromospheric activity levels of the components. We employed medium-resolution echelle spectra obtained at the 2.15m telescope at the Argentinian observatory CASLEO and photometric observations obtained from the ASAS database. 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=1.87844days), minimum masses (M₁(sini)³=0.35M_☉ and M₂(sini)³=0.33M_☉), 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 800days. This periodicity is also found in the flux of the Ca II K lines of both components, which seem to be in phase. 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 ≃2.2years (≃800days) is consistent with previous determinations of activity cycles in similar stars.