HRD with our sample stars. The symbols are as follows: Triangles represent the SCh stars, squares the CCh Miras, circles the MCh Miras, and the rectangle represents the constant Mira T Her. Open symbols are the Tc-poor stars, filled symbols Tc-rich. The gray square is W Dra, for which no definite conclusion on its Tc content could be drawn. Stars discussed individually in Sect. 4.1 are identified with their names.