The barium stars in the Hertzsprung-Russell diagram ?;??;???

We present absolute magnitudes for a sample of 52 barium stars observed by the HIPPARCOS satellite, and their location in the HR diagram. Our plot (Fig. 1) is restricted to stars with parallax accuracies better than 22 %. The luminos- ity classes range from Ib supergiants down to V dwarfs on the main sequence, as expected from spectral classication. Dis- crepancies are however noted. No gap is observed in the region extending from the main sequence to the giant branch, exactly asshownbyPerrymanetal.(1995)fornormalstars.Thisisalso true for class II bright giants. A clump is however obvious at G8-K0 IIIb and Mv 0:85 which corresponds to the one noted at (B V) 1:0 and MHp 1:0 by Perryman et al. It appears that barium stars on the main sequence are earlier than G4, up- ward evolution being noticeable for later types. They are also distributed in the subgiant zone following the locus of normal stars,i.e.increasingbrightnessforlatertypes.Afewstarsinour samplearealsoclassiedasCHstars:fourofthemaredenitely main sequence class V-dwarfs, one is a class IVb faint subgiant while two possible CH-stars are class III-giants. These results are consistent with the currently-admitted model of surface pol- lution of a normal star through mass transfer in a binary system whose primary has become a white dwarf (WD). HIPPARCOS data show perturbations of the astrometric solution which can be attributed to proved (or possible) binarity for 21 stars out of 121, and 8 of them were already quoted in the CCDM catalogue (not necessarily with a WD component). This low proportion can be explained by the 5-11 magnitudes differences predicted between the two components and/or low angular separations with periods close to one year.