A CORAVEL radial-velocity monitoring of S stars: Symbiotic activity vs. orbital separation. III.

Orbital elements are presented for the Tc-poor S stars HR 363 (= HD 7351) and HD 191226. With an orbital period of 4592 d (= 12.6 y), HR 363 has the longest period known among S stars, and yet it is a strong X-ray source. Its X-ray flux is similar to that of HD 35155, an S star with one of the shortest orbital periods (640 d). This surprising result is put in perspective with other diagnostics of binary interaction observed in binary S stars. They reveal that there is no correlation between the level of binary interaction and the orbital period. All these activity diagnostics moreover exhibit a strong time-variability. In the well-documented case of HR 1105, this time-variability appears to be a combination of orbital modulation and secular variation. A stream of gas from the red-giant wind, which is heated when funneled through the inner Lagrangian point, has been proposed as the source of the hard photons (Shcherbakov & Tuominen 1992). Different viewing angles of the stream during the orbital cycle account for the orbital modulation, whereas long-term fluctuations of the mass-loss rate account for the secular variations. Little dependence to the orbital separation is expected for this kind of activity. If such streams are causing the activity observed in the other binary S stars as well, it would provide a natural explanation for the absence of correlation between orbital periods and activity levels, since the red-giant mass loss rate would be the dominant factor. The existence of such funneled streams is moreover predicted by smooth particle hydrodynamics simulations of mass transfer in detached binary systems.