Optical studies of gamma-ray binaries and candidate systems

Highly accurate photometry of the optical companion in $\gamma$-ray binary systems has the potential to enable the exploration of previously unknown phenomena. Here we report the discovery of repeated optical flares evolving on time scales of about one day in the optical light curve of the well known system LS I +61 303. Their amplitude does not exceed 0.01-0.02 magnitudes and, therefore, they are only within reach of space observatories such as the Transiting Exoplanet Survey Satellite (TESS) in the 600-1000 nm bandpass. We tentatively propose that these flaring events are shock-powered in nature as the compact object in LS I +61303 interacts with the circumstellar envelope of its Be star companion. TESS results dealing with other systems (MWC 148, MWC 656 and HD 3191) are also shortly addressed.