  Recent studies indicate that some Galactic open clusters (OCs) exhibit
extended main-sequence turnoff (eMSTO) in their colour-magnitude diagrams
(CMDs). However, the number of Galactic OCs with eMSTO structures detected so
far is limited, and the reasons for their formation are still unclear. This
work identifies 26 Galactic OCs with undiscovered eMSTOs and investigates the
causes of these features. Stellar population types and fundamental parameters
of cluster samples are acquired using CMD fitting methods. Among them, the
results of 11 OCs are reliable as the observed CMDs are well-reproduced. We
propose the crucial role of stellar binarity and confirm the importance of
stellar rotation in reproducing eMSTO morphologies. The results also show that
the impact of age spread is important, as it can adequately explain the
structure of young OCs and fit the observed CMDs of intermediate-age OCs
better.
