{δ} MUS Revisited:Rectifying an 82 Yr-Old Mistake

The red giant ${\delta}$ Muscae is known since 1919 to be a spectroscopic binary, and the first and only orbit was determined in 1936, claiming the period to be 847 days. This was discrepant with the Hipparcos determined astrometric orbit. Using the latest data available for this object - leading to a 100 yr time-span - we show here that the correct period is 423 d, and are able for the first time to combine the spectroscopic orbit with the Hipparcos orbit. Using all the available information, we find that the $\sim$ 1.2 solar mass red giant must have a $\sim$ 0.3 - 0.4 solar mass M dwarf companion, and that the system will soon evolve towards a He WD binary system. Given its relatively short period, ${\delta}$ Muscae may be an ideal benchmark for testing astrometric orbits obtained by Gaia for very bright stars.