  The shaping of various morphological features of planetary nebulae (PNe) is
increasingly linked to the role of binary central stars. Identifying a binary
within a PN offers a powerful tool with which to directly investigate the
formation mechanisms behind these features. The Etched Hourglass Nebula, MyCn
18, is the archetype for several binary-linked morphological features, yet it
has no identified binary nucleus. It has the fastest jets seen in a PN of 630
km s$^{-1}$, a central star position offset from the nebula centre, and a
bipolar nebula with a very narrow waist. Here we report on the Southern African
Large Telescope (SALT) High Resolution Spectrograph (HRS) detection of radial
velocity variability in the nucleus of MyCn 18 with an orbital period of
$18.15\pm0.04$ days and a semi-amplitude of $11.0\pm0.3$ km s$^{-1}$. Adopting
an orbital inclination of $38\pm5$ deg and a primary mass of $0.6\pm0.1$
$M_\odot$ yields a secondary mass of $0.19\pm0.05$ $M_\odot$ corresponding to
an M5V companion. The detached nature of the binary rules out a classical nova
(CN) as the origin of the jets or the offset central star as hypothesised in
the literature. Furthermore, scenarios that produce the offset central star
during the AGB and that form narrow waist bipolar nebulae result in orbital
separations 80--800 times larger than observed in MyCn 18. The inner hourglass
and jets may have formed from part of the common envelope ejecta that remained
bound to the binary system in a circumbinary disk, whereas the offset central
star position may best be explained by proper motion. Detailed simulations of
MyCn 18 are encouraged that are compatible with the binary nucleus to further
investigate its complex formation history.
