O–C diagram for the dominant period of OGLE-BLAP-001 plotted as a function of time. The observed times of maximum light (O) and epochs E are given in Table A.1. The predicted times of maximum light (C) were calculated according to Eq. (1). The sources of data are labelled in the plot. The dashed line stands for smoothed changes, which were used to correct the times of observations for period changes, prior to the final analysis presented in Sect. 4.1.5.