Colour–magnitude diagram of X-1 from HST/WFC3. The optical counterpart candidate (c1) is shown as colourful stars in different observations, and field stars within 7″ around X-1 are shown as grey circles. The black, red, and blue colours represent magnitudes of c1 calculated using H4, H6, and H8 data, respectively. The PADOVA isochrones with different ages have been corrected for extinction of A_V = 0.46 mag.