This paper addresses the question of why metal-deficient barium stars are not yellow symbiotic stars ( YSyS ) .
Samples of ( suspected ) metal-deficient barium ( mdBa ) stars and YSyS have been collected from the literature , and their properties reviewed .
It appears in particular that the barium nature of the suspected mdBa stars needs to be ascertained by detailed abundance analyses .
Abundances are therefore derived for two of them , HD 139409 and HD 148897 , which reveal that HD 148897 should not be considered a barium star .
HD 139409 is a mild barium star , with overabundances observed only for elements belonging to the first s-process peak ( Y and Zr ) .
It is only moderately metal-poor ( [ Fe/H ] = -0.4 ) .
The evidence for binarity among metal-deficient barium stars is then reviewed , using three different methods : ( i ) radial-velocity variations ( from CORAVEL observations ) , ( ii ) Hipparcos astrometric data , and ( iii ) a method based on the comparison between the Hipparcos and Tycho-2 proper motions .
An orbit is obtained for HIP 55852 , whereas evidence for the ( so far unknown ) binary nature of HIP 34795 , HIP 76605 , HIP 97874 and HIP 107478 is presented .
No conclusion regarding the binary nature of HIP 11595 , HIP 25161 could be reached .
Two stars with no evidence for binarity whatsoever ( HIP 58596 and BD +3 ^ { \circ } 2688 ) are candidates low-metallicity thermally-pulsing asymptotic giant branch stars , as inferred from their large luminosities .
The reason why mdBa stars are not YSyS is suggested to lie in their different orbital period distributions : mdBa stars have on average longer orbital periods than YSyS , and hence their companion accretes matter at a lower rate , for a given mass loss rate of the giant star .
The definite validation of this explanation should nevertheless await the determination of the orbital periods for the many mdBa stars still lacking periods , in order to make the comparison more significant .