Analysis of the time series of three M dwarfs exhibiting new long-term cyclic modulation. Upper panels: time series for each star. The color coding and markers denote the instrument (see legend to the right) and the red dashed line is the best-fit sine function. Middle panels: residuals after subtraction of the best fit. Bottom panels: GLS periodogram of the time series. The vertical dashed line marks the period with the highest power.