Detection of a BY Draconis-like binary companion to contact binary XY Leonis

Spectroscopic observations are presented which reveal the presence of the previously suspected companion of the well-known W UMa binary, XY Leo. This companion is deduced to be a BY Draconis binary composed of late-K to mid-M dwarf stars with an orbital period of 0.d805. The Hα emission features presented here arise from the BY Draconis component, not from the contact binary. The Ca II H and K emission features reported by previous investigators to be attributable to the more massive W UMa component are, instead, associated with the components of the BY Draconis binary. This finding has obvious implications on the use of XY Leonis in the study of the chromospheric activity relations among close binary systems. Either high-resolution spectroscopy (λ/Δλ > 10,000) or high spatial resolution imaging (< 0."1) will be required to properly disentangle the chromospheric and coronal emission arising from each binary component. Once proper observation and deconvolution is possible XY Leonis should prove to be valuable for such studies, given the equal ages and chemical compositions of the two binaries, and since the masses and inclinations for each component will be well determined for this system as soon as the astrometric orbit is measured. In the present study, self-consistent arguments suggest non-coplanarity in the orbits.