Photometric index S_ph (percent) as a function of rotation period P_rot (days) for the 86 flaring stars from Table B.1. Both axes are in logarithmic scale. The blue and red symbols indicate the non-oscillating and oscillating giant stars, respectively. The orange markers indistinctly indicate dwarf and subgiant stars. The upward-pointing triangles are confirmed close spectroscopic binaries including at least one red giant, and the circles identify RV-stable red giant stars (Gaulme et al. 2020). The orange diamond with black edges is a confirmed subgiant belonging to a close binary system (KIC-11551404, Gaulme et al. 2020). The gray background reflects the distribution of S_ph of the regular inactive red giant stars (the darker, the higher) according to Gaulme et al. (2020).