Observed (O) minus calculated (C) epochs for primary minima versus cycle number for 117 years of observations, according to the ephemeris given by Eq. (1). Black and red dots indicate photographic and visual data, respectively. Blue, green, and magenta dots show Charge-Coupled Device (CCD) or photoelectric V-, I_c-, and g-band data, respectively. The data points with the same X and Y coordinates are displayed with a small X offset for better visualization. See Sect. 3.1.