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We studied the polarised spectrum of the post-AGB binary system 89\,Herculison the basis of data collected with the high resolution \emph{CataniaAstrophysical Observatory Spectropolarimeter}, \emph{HArps-North POlarimeter}and \emph{Echelle SpectroPolarimetric Device for the Observation of Stars}. Wefind the existence of linear polarisation in the strongest metal lines inabsorption and with low excitation potentials. Signals are characterized bycomplex Q and U morphologies varying with the orbital period. As possibleorigin of this "Second Solar Spectrum"-like behaviour, we rule out magneticfields, continuum depolarisation due pulsations and hot spots. { The linearpolarisation we detected also in the Ca{\sc ii}\,8662\AA\, line is a clearevidence of optical pumping polarisation and it rules out the scatteringpolarisation from free electrons of the circumbinary environment.} In theframework of optical pumping due to the secondary star, the observed periodicproperties of the spectral line polarisation can be justied by two jets, flowvelocity of few tens of km\,s$^{-1}$, at the basis of that hour-glass structurecharacterising 89\,Herculis. We also discovered linear polarisation across theemission profile of metal lines. Numerical simulations show that thesepolarised profiles could be formed in an undisrupted circumbinary disk rotatingat $\le$10 km\,s$^{-1}$ and whose orientation in the sky is in agreement withoptical and radio interferometric results. We conclude that the study of thoseaspherical enevlopes, whose origin is not yet completely understood, of PNe andalready present in the post-AGB's, can benefit of high resolutionspectropolarimetry and that this technique can shape envelopes still too farfor interferometry.

The solar-like "Second Spectrum" and the polarised metal lines in emission of the post-AGB binary 89 Herculis