  HIPPARCOS trigonometrical parallaxes make it possible to compare the location
of Tc-rich and Tc-poor S stars in the Hertzsprung-Russell (HR) diagram: Tc-rich
S stars are found to be cooler and intrinsically brighter than Tc-poor S stars.
The comparison with the Geneva evolutionary tracks reveals that the line
marking the onset of thermal pulses on the asymptotic giant branch (AGB)
matches well the observed limit between Tc-poor and Tc-rich S stars. Tc-rich S
stars are, as expected, identified with thermally-pulsing AGB stars of low and
intermediate masses, whereas Tc-poor S stars comprise mostly low-mass stars
(with the exception of 57 Peg) located either on the red giant branch or on the
early AGB. Like barium stars, Tc-poor S stars are known to belong exclusively
to binary systems, and their location in the HR diagram is consistent with the
average mass of 1.6 +- 0.2 Msun derived from their orbital mass-function
distribution. A comparison with the S stars identified in the Magellanic Clouds
and in the Fornax dwarf elliptical galaxy reveals that they have luminosities
similar to the galactic Tc-rich S stars. However, most of the surveys of S
stars in the external systems did not reach the lower luminosities at which
galactic Tc-poor S stars are found. The deep Westerlund survey of carbon stars
in the SMC uncovered a family of faint carbon stars that may be the analogues
of the low-luminosity, galactic Tc-poor S stars.
