Contribution \`{a} l'etude des binaires des types F, G, K, M IX. HD 191588, nouvelle binaire spectroscopique \`{a} raies simples de type RS Cvn, systeme triple

An accident of misidentification has brought to light the interesting system HD 191588, a new RS CVn-type spectroscopic binary. A radial-velocity study of the primary star, the only seen component, carried out at the Observatoire de Haute-Provence with the Coravel instrument and subsequently at the Cambridge Observatories with a similar one, reveals two orbital motions: a short-period orbit (60 days) and a long-period one (about 4.5 years), so this star is a triple system. The following orbital elements are obtained: (1) for the long-period orbit P = 1667+/-17 days, T = 50901 +/-67 MJD, Gamma = +2.09 +/-0.07 km/s, K = 2.51 +/-0.13 km/s, e = 0.18 +/-0.04, omega = 228deg +/- 14 deg, a1 sin i = 56.7 +/- 3.0 Gm, f(m) = 0.0026 +/-0.0004 M_sun, and (2) for the short-period orbit P = 60.0269 +/-0.0016 days, T = 50482.6 +/-3.3 MJD, gamma is var., K = 24.03 +/- 0.09 km/s, e = 0.012 +/-0.004, omega = 233 deg +/-19deg, a1 sin i = 19.83 +/-0.07 Gm, f(m) = 0.0865 +/-0.0009 M_sun. From near-infrared observations we refine the classification of the primary component and we found a spectral type of K2.5 III, and a spectrum obtained in the blue--near-UV spectral region reveals strong H and K emission lines of Ca II. The unseen secondary should be a solar-type star (F or G V); the minimum mass of the third body is that of a dwarf M star. Probably, the primary component rotates in synchronism with the orbital motion in the inner orbit; a model, based upon that hypothesis, is proposed for the system, and finally the connection of the inner binary to the long-period RS CVn group (Hall 1976) is discussed.