Evolutionary track that passes through the region of the surface gravity-effective temperature plane where the four sdO stars of interest are located. Those are depicted by red crosses. Each available model along the track is indicated by a small black dot. Model #7, indicated by a large open magenta circle, is representative of stars in the EHB phase of evolution and is taken as a reference. Model #21, designated by a large open blue circle, indicates the end of the He-core burning phase. Model #25, shown by a large open red circle, is representative of the post-EHB stars of interest.