Upper panel: star count map of Do I showing the stars detected in both g and r-bands in our GTC observations. Lower panel: de-reddened (g − r)₀ vs. r₀ color-magnitude diagram of Do I (panel a) and its control field (panel b). Panel c shows a comparison of the Do I CMD with the PARSEC isochrones (solid lines) with an age of 13 Gyr shifted assuming a distance modulus of (m − M)=27.6 mag. The metallicities of the isochones are Z = 0.0001 (cyan), 0.0003 (red), 0.0005 (blue) and 0.0007 (green), respectively.