O–C diagrams for GSN 069. We show the O–C diagrams for odd (left) and even (right) QPEs for GSN 069 resulting from identifying the first QPE of the May 2019 observation with the 211th even QPE. The upper panels show the O–C data together with the linear plus parabolic baseline model for P_mod ≃ 19 d. The lower panels show the corresponding residuals (O–C_BASELINE) as well as the ones corresponding to the full best-fitting model including a sinusoidal modulation (O–C_FULL) for the two possible P_mod. The sinusoidal modulation is also shown in the O–C_BASELINE to guide the eye.