(Abridged) The origin of hot subdwarf O-type stars (sdOs) remains unclearsince their discovery in 1947. Among others, a post-Asymptotic Giant Branch(post-AGB) origin is possible for a fraction of sdOs. We are involved in acomprehensive ongoing study to search for and to analyze planetary nebulae(PNe) around sdOs with the aim of establishing the fraction and properties ofsdOs with a post-AGB origin. We use deep Halpha and [OIII] images of sdOs todetect nebular emission and intermediate resolution, long-slit opticalspectroscopy of the detected nebulae and their sdO central stars. These dataare complemented with other observations for further analysis of the detectednebulae. We report the detection of an extremely faint, complex PN around 2MASSJ19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system.The PN shows a bipolar and an elliptical shell, whose major axes are orientedperpendicular to each other, and high-excitation structures outside the twoshells. WISE archive images show faint, extended emission at 12 and 22 micronsin the inner nebular regions. The internal nebular kinematics is consistentwith a bipolar and a cylindrical/ellipsoidal shell, in both cases with the mainaxis mainly perpendicular to the line of sight. The nebular spectrum onlyexhibits Halpha, Hbeta and [OIII]4959,5007 emission lines, but suggests a verylow-excitation ([OIII]/Hbeta = 1.5), in strong contrast with the absence oflow-excitation emission lines. The spectrum of 2M1931+4324 presents narrow,ionized helium absorptions that confirm the previous sdO classification andsuggest an effective temperature >= 60000 K. The binary nature of 2M1931+4324,its association with a complex PN, and several properties of the system providestrong support for the idea that binary central stars are a crucial ingredientin the formation of complex PNe.
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