ON THE REFLECTION EFFECT IN 3 SDOB BINARY STARS

Echelle spectroscopy is presented of three binary systems containing h ot subdwarf primary stars and low-mass, low-luminosity secondary stars - HW Vir, AA Dor and KV Vel. The semi-amplitudes of the radial-veloci ty curves of the primary components are determined to an accuracy of 1 - 2 per cent. The secondary component of KV Vel displays a rich set o f weak emission lines due to CII, CIII, CIV, NII, NIII and OII, arisin g from the hemisphere that is irradiated by the hot primary star. For HW Vir and AA Dor, an extensive search, via proven cross-correlation p rocedures, fails to reveal any spectral signatures attributable to the secondary components. The published multi-colour light curves of thes e three systems are analysed to confirm some previous results, and to demonstrate that the amplitude and detailed shape of the reflection ef fect can be matched provided that the irradiated hemispheres of the (o therwise) cool secondary stars have bolometric albedos of unity, and h ave limb-darkening coefficients much reduced over normal values, or ev en exhibit limb-brightening. Astrophysical parameters are determined, based upon these analyses and assuming, in the cases of HW Vir and AA Dor, a mass of 0.5 M circle dot for the subdwarf stars. The three hot subdwarfs lie in the region of the HR diagram appropriate for evolutio n models for binary systems undergoing late case B mass transfer follo wed by a common-envelope phase. For HW Vir and AA Dor, the cool second ary stars are entirely consistent with ZAMS models at 0.09-0.14M circl e dot, whilst the secondary in KV Vel has logR/R circle dot similar to 0.2 dex larger than a ZAMS model for its mass of 0.23 M circle dot.