New X-ray sources detected among mild barium and S stars.

We report on the detection by ROSAT of X-rays from the mild barium star HD 165141 (K0III/IIBa1) and from the S stars HD 35155 (S4,1) and HR 363 (S3/2). For the S stars, the X-ray flux is attributed to the accretion of the red-giant wind by the white dwarf companion. The strong variability observed in the X- and UV fluxes on time scales of both hours and months may be due to irregularities in the accretion rate, or to variable obscuration by cool gas present in the system. In the case of HD 35155, the absence of ROSAT detection at the phase where eclipses are observed in the UV and optical domains suggests that part of the X-ray variability may be associated with eclipses of the compact companion. HD 165141 is more puzzling since this star seems to share the properties of RS CVn and barium systems. The properties of the X-rays emitted by this system are typical of RS CVn systems, as is the photometric period of 35d and the rapid rotation. However, the rapid rotation does not seem to be due to synchronism with the orbital period, as is usually the case for RS CVn systems. The companion appears to be a hot white dwarf rather than a main sequence star, with a long orbital period (~5200d), more typical of barium than of RS CVn systems. These conflicting properties could be explained if this particular barium star formed on the giant branch, accreting not only mass but also spin angular momentum. The two giants HR 5692 (G8IIIBa0.3) and HR 6468 (G8IIIBa0.6) appear to be coronal X-ray sources. The barium nature of these stars is questioned, given their small Ba indices and their normal DDO photometric indices. Moreover, since HR 6468 is a radial-velocity standard star, it is likely not a binary star as required for barium stars.