The Henize sample of S stars III. Uncovering the binary intruders

The properties of S stars are investigated thanks to a large observing prog ram devoted to the well-defined Henize sample (205 S stars south of delta = -25 degrees and brighter than R = 10.5, covering all galactic latitudes), in order to derive the respective properties of the intrinsic S stars (genu ine thermally-pulsing AGE stars) and of the extrinsic S stars (post mass-tr ansfer binaries). The stellar sample is first cleaned from a few stars misclassified as S tha nks to UBV Geneva photometry and low-resolution spectroscopy. These low-res olution spectra also allow to successfully distinguish subclasses within th e S star family. Dedicated Geneva photometry and high-resolution spectrosco py have led to the discovery of two symbiotic S stars. The more stringent difference between extrinsic and intrinsic stars is thei r technetium content, but several other observational parameters are shown to be efficient to some extent in segregating intrinsic S stars from their extrinsic masqueraders (UBV, JHKL and IRAS photometry, radial-velocity stan dard deviation, shape of the CORAVEL cross-correlation dip, combination of band strength indices derived from low-resolution spectra). Multivariate cl assification has been performed on the Henize data sample in order to guara ntee a classification as objective as possible and handling at the same tim e a large number of parameters. The resulting clusters separate efficiently extrinsic and intrinsic S stars, allowing to derive the respective propert ies of these two distinct stellar classes. The population difference betwee n intrinsic and extrinsic S stars is for the first time clearly demonstrate d, since intrinsic S stars are far more concentrated towards the galactic p lane than extrinsic S stars (z(int) = 200 +/- 100 pc and z(ext) = 600 +/- 1 00 pc), and are therefore believed to belong to a younger, more massive pop ulation. The frequency of extrinsic and intrinsic S stars in the magnitude- limited Henize sample amounts to 33% and 67%, respectively. In a volume-lim ited sample, this proportion is subject to large uncertainties mainly becau se of uncertain luminosities. There are probably as many as 40% extrinsic s tars among S stars in a volume-limited sample.