Ground-based photometric observations (left) and generalized Lomb-Scargle (GLS, Zechmeister & Kürster 2009) periodograms (right) for each dataset of GJ 806. Fitting a linear function and a kernel with a periodic term to all the datasets using GPs, we find a stellar rotation period of days. The dashed vertical blue line marks the 13.6-day period discussed in later sections. The horizontal dotted red line is the 10% false alarm probability level.