Panel a: combined S_MWO time series is displayed with black crosses. The red asterisks show the seasonal data from SSS and TIGRE data and the solid line the result of the linear trend fit. The dotted line depicts the sinusoidal fit of the Lomb-Scargle analysis of the non-detrended data with a period of 3.91 yrs. Panel b: detrended S_MWO time series. The solid line represents the sinusoidal fit with the period of 3.88 yr found in the GLS analysis. Panel c: periodograms of GLS (generalised Lomb-Scargle) analysisof detrended and non-detrended data. Panel d: phase-folded detrended S_MWO time series shown with sinusoidal fit. Here, the S_MWO of the individual time series are colour-coded and labelled (Keck: green asterisks, SMARTS: red diamonds, SSS: black crosses, and TIGRE: blue triangles).