m_F336W vs. m_F336W − m_F814W (left) and m_F336W vs. m_F275W − m_F336W (right) CMDs of stars in the NGC 1850 cluster field. The CMDs are zoomed-in around the luminosity interval that we used to investigate the incidence of binaries among the stellar populations with different rotation rates. The probable rMS and bMS stars, selected in the left-panel CMD, are colored red and blue, respectively, whereas the remaining stars are represented with gray dots. The insets show the fiducial lines of the rMS and bMS (continuous lines), and the fiducial lines of equal-luminosity bMS and rMS binaries (dashed lines).