Magnetic Cycles of Oscillating Red Giants in Eclipsing Binaries

Although magnetic activity cycles are well-established in main sequence stars, red giant branch magnetic activity cycles have not been demonstrated to date. Indeed, most red giants are too slowly rotating to characterize, but giants with short rotation periods due to binary interactions offer readily detectable rotation rates, with magnetic field variability potentially detectable using asteroseismology. If the dynamo mechanism operates in red giants as it does in main sequence stars, there should be a relationship between these rotation periods and magnetic cycle period. Here, we investigate potential magnetic cycle variability from observed variations in ν_max and △ν from Kepler among a handful of rapidly-rotating giants in binary systems. We also present analysis of asteroseismic variability from TESS, which significantly extends the time baseline. With this data, we place limits on the magnetic cycle periods for these stars, situating them in a period-cycle diagram. A subset exhibit asteroseismic variability on timescales longer than the rotation period but shorter than the expected magnetic cycle period from extrapolations of main sequence stars, which may be analogous to biennial solar magnetic field variability. The present analysis offers motivation for continued monitoring of Kepler targets with TESS and PLATO missions to provide constraints on decades-long magnetic cycles in giants.