1998AJ....115.2028T

We report the serendipitous discovery of an Algol-type binary with an orbital period of 18.9 days, in which both components are cool giants (G8 III and K1-K2 III). Few systems with these characteristics are known and are referred to as ``cool Algols'', as opposed to the ``classical Algols'' in which the mass-gaining component resembles an early-type main-sequence star. The nature of this object, which is a strong X-ray source, was discovered in the course of a spectroscopic search for T Tauri stars among ROSAT All-Sky Survey sources. We derive a double-lined spectroscopic orbit leading to minimum masses of M_A = 2.481±0.023 M_☉ and M_B = 0.515±0.011 M_☉ and a corresponding mass ratio of q ∼ 0.21. We measure also projected rotational velocities of 22 and 31 km.s^–1, respectively, and a luminosity ratio of l_B/l_A = 1.06±0.04 at ∼5200 Å. The system is probably semidetached, with the axial spin of the components synchronized with the orbital motion, and the probability of eclipses is high. The general properties of BD +05 706, most likely a result of case B mass exchange, are very similar to those of the other known members of this class. We discuss the correlation between the mass of the hotter, detached components and their effective temperature for all cool Algols, noted previously by D. M. Popper, as well as a trend with the orbital period. Chromospheric activity indicators in BD +05 706 are compared with those of other members of its class, as well as with those of the classical Algols and the RS CVn binaries.