Seasonal periodograms of residual time series obtained after fitting the RV time series with four-planet models. From top to bottom, the rows are for seasons 2008, 2009, and 2010+2011, respectively. From left to right, the columns are periodograms to investigate signals b, (e), c, and (f), respectively. The periodicity of each signal is located with a vertical dashed red line. Power excesses are seen at all seasons for signals b and c, but not for signals (e) and (f).