Modified Hertzsprung-Russell diagram showing the stellar bulk density versus effective temperature. The position of Kepler-423 is over-plotted on theoretical evolutionary tracks and isochrones from the Pisa Stellar Evolution Data Base for low-mass stars. The blue hatched areas represent different masses (0.81, 0.85, and 0.89 M_⊙ from right to left), while the greys represent the age isochrones (9, 11, and 13 Gyr from bottom to top), computed for an initial metal content between Z = 0.012 and Z = 0.014.