Graphic representation of the three kernels used in this study: the quasi-periodic (QP) kernel as implemented in the george code (top), the simple harmonic oscillator (SHO) as implemented in the celerite code (middle panel), and the quasi-periodic and cosine (QPC, bottom) kernel introduced in this work, for P = 25 days. The x-axis is the time-lag τ between two data points, and the vertical axis is the normalised value of the correlation represented by the kernel. We show the kernels in red for λ = 25 days (w = 0.31) and black forλ = 75 days. For the QP kernel, the light grey area marks 0 < w < 1, whereas the dark grey area marks the interval 0.2 < w < 0.5, which is more physically motivated. The QPC kernel has h₂∕h₁ = 1.