Location of the three stars in the HR diagram. The solid lines are the theoretical evolutionary models of Blöcker (1995b), labelled with the core mass in solar units. The long-dashed lines show the location of the stars based on the V-band magnitude: from right- to left-hand side He 2-436, Wray 16-423 and StWr 2-21. The square symbols show the specific models of Clegg & Middlemass (1987) used in deriving these lines. The photoionization models are shown by the plus signs: in all cases they are on or to the left-hand side of the corresponding long-dashed line.