SPECTROSCOPY AND ECLIPSE-MAPPING OF THE MASS-EXCHANGING BINARY STAR V361 LYR

We present the first spectroscopic investigation of the short-period ( P = 0.309614 d) mass-exchange binary system V361 Lyr. From observation s over two complete orbits, we show that he system is double-lined, wi th the stars having minimum masses of m(1)sin(3)i = 1.25 +/- 0.03 M. a nd m(2)sin(3)i = 0.87 +/- 0.03 M.. The mass ratio m(2)/m(1) = 0.69 +/- 0.02 and the projected semimajor axis of the relative orbit is a sin i = 2.47 +/- 0.05 R.. The spectral type of the primary component is es timated to be F8-G0. The H alpha line profile clearly shows two absorp tion components when the system is at its brightest and bluest stage, around orbital phases 0.35-0.45. The published V and I light curves ar e analysed with LIGHT2 to show that the primary component (T = 6200 K) fills its Roche lobe, whilst the secondary (T = 4500 K) is detached a nd fills approximate to 57 per cent of the available Roche volume. Ecl ipse maps of both stellar surfaces are derived via DOTS from light cur ves obtained in 1988, 1989 and 1992. These show that the secondary has a persistent elongated hot structure on its equator with an estimated temperature of 10000 K that is consistent with the expectations of a theoretical mass transfer stream leaving the primary at its inner Lagr angian point and falling on to the secondary. The accretion luminosity of approximate to 0.5 L. from that impacting stream, as estimated fro m the observed decrease in orbital period and the derived stellar mass es, it about twice the luminosity of the structure inferred from the l ight curves and the surface-image reconstructions. The eclipse maps fo r the primary illustrate small changes in the cool spot structures tha t are typical of magnetic activity observed in other rapidly-rotating solar-type stars that are single or are members of binary systems (e.g . AB Dor, XY UMa). This model for V361 Lyr predicts successfully the p hase-dependent variations observed in the H alpha line. The properties of this mass-exchanging semidetached binary are compared with the nea r-contact and the contact systems of the W UMa type for the purpose of establishing whether or not it represents the short-lived phase of ev olution into contact for the first time, or the broken-contact stage o f a thermal-relaxation oscillation. The derived locations of both comp onents on the zero-age main sequence would favour the former interpret ation. V361 Lyr is reddened by E(B - V) = 0.43 and lies at a distance of 520 +/- 40 pc.