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 \pm 0.04 days and a semi-amplitude of 11.0 \pm 0.3 km s ^ { -1 } .
Adopting an orbital inclination of 38 \pm 5 deg and a primary mass of 0.6 \pm 0.1 M _ { \odot } yields a secondary mass of 0.19 \pm 0.05 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 .