Top left: comparison between the observed CMD of NGC 1844 and suitable theoretical isochrones for an age of 150 Myr and 90 Myr for two different assumptions about the heavy-element mixtures, an α-enhanced mixture ([α/Fe] =  + 0.4, solid blue line), and a CNO-enhanced mixture (dashed red line), both with the same iron content [Fe/H] = −0.6. In this comparison we assumed (m − M)_F814W = 18.52 and E(m_F475W − m_F814W) = 0.15 for both isochrones. Top-right: in this case the two isochrones have the same α-enhanced mixture but different metallicities, namely Z = 0.01 and Z = 0.015. Bottom left: in this case the two isochrones have the same metallicity, Z = 0.008, but two different initial He abundances. In this last case we adopted (m − M)_F814W = 18.40. Bottom-right: comparison between the NGC 1844 MS and the same isochrone (α-enhanced, 150 Myr, Z = 0.01) adopted in the top left panel (solid blue line). To investigate the impact of stellar rotation, we also show the same isochrone modified according to the Bastian & De Mink (2009) recipes to take into account the effect of rotation (dashed red line). (See text for more detail).