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The nearby K dwarf HD 98800 is remarkable for its unusual infrared excess. The radiation between 10 and 100 microns amounts to more than 10 percent of the luminosity of the star, indicating a large disc of cold dust surrounding the star. A visual companion was discovered by Innes in 1909, and has moved about 1.5 arcsec north over the past 85 years, crossing the primary almost exactly. If physically connected, the orbit of the companion imposes some interesting geometrical constraints on the location and size of the disc.
Based on 174 spectra obtained with the CfA Digital Speedometers, we find that the visual primary is a single-lined spectroscopic binary with period 265 days, amplitude 6.5 km/s, and eccentricity 0.49, implying a low-mass stellar companion. The visual companion is a double-lined spectroscopic binary with period 314 days, eccentricity 0.7, and mass ratio 0.9. Assuming that the two spectroscopic binaries are physically bound, we can derive an orbit by combining the difference in the center-of-mass velocities with the visual positions. The period comes out to about 800 years with a semi-major axis of 130 AU and eccentricity of about 0.7. This orbit is relatively insensitive to the assumed distance. At closest approach the two binaries are separated by only 30 AU, while at ap-system they are almost 200 AU apart. Does this leave room for a dust disc? On the other hand, there is not yet any proof that the two binaries are physically connected. In that case there should be plenty of room for a dust disk around either of the spectroscopic binaries.
