Upper panel: evolutionary tracks of helium stars produced in binary systems through Case AB or B mass transfer phase, and of a single helium star of 3 M_⊙ (black solid line), at solar metallicity. The star symbol marks the end point of each evolutionary track, which is the core carbon burning phase for 3 M_⊙ star and the oxygen burning phase for the others. The helium-star models in binary systems are taken from Yoon et al. (2010), and the evolutionary tracks are plotted only from the onset of core helium burning. The sequence number given for each track (except for the 3 M_⊙ single helium star) denotes the model sequence number in Table 1 of Yoon et al. (2010). The final mass (M_f) for each model sequence is given in the parenthesis. Lower panel: corresponding absolute narrow-band visual magnitudes calculated with the assumption of black-body radiation.