Context. It is now clear that binarity plays a crucial role in many aspects of planetary nebulae (PNe), particularly the striking morphologies that they show. To date, there are ∼ 60 bCSPNe known. However, both theory and observation indicates that this represents only the tip of the iceberg, with the Galactic PN population hosting orders of magnitude more. Aims. We are involved in a search for new bCSPNe to enhance the statistical validation of the key role of binarity in the formation and shaping of PNe. New discoveries of bCSPNe and their characterization have important implications not only in understanding PN evolution but also in understanding binary evolution and the poorly-understood common-envelope phase. Methods. We used data from the TESS satellite to search for variability in the eight CSPNe that belong to the two-minute cadence preselected targets in Cycle 1, which have available pipeline-extracted light curves. We identified strong periodicities and analysed them in the context of the binary scenario. Results. All the CSPNe but one (Abell 15) show clear signs of periodic variability in TESS. The cause of this variability can be attributed to different effects, some of them requiring the presence of a companion star. We find simple sinusoidal modulations in several of the systems, compatible to irradiation effects. In addition, two of the central stars (PG 1034+001 and NGC 5189) also show photometric variations due to ellipsoidal variations and other signs of variability probably caused by star spots and/or relativistic Doppler-beaming. Especially interesting is the case of the well-studied Helix Nebula, in which we constructed a series of binary models to explain the modulations we see in the light curve. We find that the variability constrains the possible companion to be very low-mass main-sequence star or sub-stellar object. We also identify with a great detail the individual pulsation frequencies of NGC 246.

The importance of long-period binaries for the formation and evolution of planetary nebulae is still rather poorly understood, which in part is due to the lack of central star systems that are known to comprise such long-period binaries. Here, we report on the latest results from the on-going Mercator-HERMES survey for variability in the central stars of planetary nebulae. We present a study of the central stars of NGC 1514, BD+30• 623, the spectrum of which shows features associated with a hot nebular progenitor as well as a possible A-type companion. Cross-correlation of high-resolution HERMES spectra against synthetic spectra shows the system to be a highly eccentric (e ∼ 0.5) double-lined binary with a period of ∼3300 days. Previous studies indicated that the cool component might be a horizontal branch star of mass ∼0.55 M , but the observed radial velocity amplitudes rule out such a low mass. If we assume that the nebular symmetry axis and binary orbital plane are perpendicular, then the data are more consistent with a post-main-sequence star ascending towards the giant branch. We also present the continued monitoring of the central star of LoTr 5, HD 112313, which has now completed one full cycle, allowing the orbital period (P∼2700 days) and eccentricity (e ∼ 0.3) to be derived. To date, the orbital periods of BD+30• 623 and HD 112313 are the longest to have been measured spectroscopically in the central stars of planetary nebulae. Furthermore, these systems, along with BD+33• 2642, comprise the only spectroscopic wide-binary central stars currently known.

We present narrow-band Hα and [O iii] images, and high-resolution, long-slit spectra of the planetary nebulae (PNe) Abell 36, DeHt 2, and RWT 152 aimed at studying their morphology and internal kinematics. These data are complemented with intermediate-resolution, long-slit spectra to describe the spectral properties of the central stars and nebulae. The morphokinematical analysis shows that Abell 36 consists of an inner spheroid and two bright pointsymmetric arcs; DeHt 2 is elliptical with protruding polar regions and a bright non-equatorial ring; and RWT 152 is bipolar. The formation of Abell 36 and DeHt 2 requires several ejection events including collimated bipolar outflows that probably are younger than and have disrupted the main shell. The nebular spectra of the three PNe show a high excitation and also suggest a possible deficiency in heavy elements in DeHt 2 and RWT 152. The spectra of the central stars strongly suggest an sdO nature and their association with PNe points out that they have most probably evolved through the asymptotic giant branch. We analyze general properties of the few known sdOs associated to PNe and find that most of them are relatively or very evolved PNe, show complex morphologies, host binary central stars, and are located at relatively high Galactic latitudes.

LoTr 5 is a planetary nebula with an unusual long-period binary central star. As far as we know, the pair consists of a rapidly rotating G-type star and a hot star, which is responsible for the ionization of the nebula. The rotation period of the G-type star is 5.95 days and the orbital period of the binary is now known to be ∼2700 days, one of the longest in central star of planetary nebulae. The spectrum of the G central star shows a complex Hα double-peaked profile which varies with very short time scales, also reported in other central stars of planetary nebulae and whose origin is still unknown. We present new radial velocity observations of the central star which allow us to confirm the orbital period for the long-period binary and discuss the possibility of a third component in the system at ∼129 days to the G star. This is complemented with the analysis of archival light curves from SuperWASP, ASAS and OMC. From the spectral fitting of the G-type star, we obtain a effective temperature of T eff = 5410±250 K and surface gravity of log g = 2.7±0.5, consistent with both giant and subgiant stars. We also present a detailed analysis of the Hα double-peaked profile and conclude that it does not present correlation with the rotation period and that the presence of an accretion disk via Roche lobe overflow is unlikely.

NGC 1514 is a complex planetary nebula with a peculiar binary central star (BD+30• 623) consisting of a cool star and a hot companion. To date, the parameters of the two stars have not been firmly stablished. We present a detailed spectral analysis of BD+30• 623 based on intermediate-resolution CAFOS optical spectra and IUE ultraviolet spectra with the goal of deriving the parameters of the two stars. For this purpose, we used an extensive composite grid of Kurucz and Tübingen NLTE Model-Atmosphere spectra. From the fitting procedure, in terms of the minimum χ 2 method, the best models obtained correspond to an HorizontalBranch A0 star with T eff = 9850±150 K, log g = 3.50±0.25, and a hot companion with T eff between 80000 K and 95000 K and a log g ≃ 5.5. To our knowledge, this is the first time that the parameters of both stars have been determined accurately through a detailed spectroscopic analysis.

Context. The origin of hot subdwarf O-type stars (sdOs) remains unclear since their discovery in 1947. Among others, a postasymptotic giant branch (post-AGB) origin is possible for a fraction of sdOs. Aims. We are involved in a comprehensive ongoing study to search for and to analyze planetary nebulae (PNe) around sdOs with the aim of establishing the fraction and properties of sdOs with a post-AGB origin.Methods. We used deep Hα and [O iii] images of sdOs to detect nebular emission and intermediate-resolution, long-slit optical spectroscopy of the detected nebulae and their sdO central stars. These data were complemented with other observations (archive images, high-resolution, long-slit spectroscopy) for further analysis of the detected nebulae. Results. We report detecting an extremely faint, complex PN around 2MASS J19310888+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 oriented perpendicular to each other, and high-excitation structures outside the two shells. WISE archive images show faint, extended emission at 12 μm and 22 μm in the inner nebular regions. The internal nebular kinematics, which is derived from high-resolution, long-slit spectra, is consistent with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main axis mainly perpendicular to the line of sight. The nebular spectrum only exhibits Hα, Hβ, and [O iii]λλ4959, 5007 emission lines, but suggests a very low-excitation ([O iii]/Hβ 1.5), in strong contrast to the absence of low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow, ionized helium absorptions that confirm the previous sdO classification and suggest an effective temperature ≥60 000 K. The binary nature of 2M1931+4324, its association with a complex PN and several properties of the system provide strong support for the idea that binary central stars are a crucial ingredient in the formation of complex PNe.

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Context. Exoplanet searches through space-based photometric time series have shown to be very efficient in recent years. However, follow-up efforts on the detected planet candidates have been demonstrated to be critical to uncover the true nature of the transiting objects. Aims. In this paper we show a detailed analysis of one of those false positives hidden as planetary signals. In this case, the candidate KOI-3886.01 showed clear evidence of a planetary nature from various techniques. Indeed, the properties of the fake planet set it among the most interesting and promising for the study of planetary evolution as the star leaves the main sequence. Methods. To unveil the true nature of this system, we present a complete set of observational techniques including high-spatial resolution imaging, high-precision photometric time series (showing eclipses, phase curve variations, and asteroseismology signals), high-resolution spectroscopy, and derived radial velocities to unveil the true nature of this planet candidate. Results. We find that KOI-3886.01 is an interesting false positive case: a hierarchical triple system composed by a ~K2 III giant star (KOI-3886 A) accompanied by a close-in eclipsing binary formed by a subgiant ~G4 IV star (KOI-3886 B) and a brown dwarf (KOI-3886 C). In particular, KOI-3886 C is one of the most irradiated brown dwarfs known to date, showing the largest radius in this substellar regime. It is also the first eclipsing brown dwarf known around an evolved star. Conclusions. In this paper we highlight the relevance of complete sets of follow-up observations to extrasolar planets detected by the transit technique using large-pixel photometers such as Kepler and TESS and, in the future, PLATO. In particular, multi-color high-spatial resolution imaging was the first hint toward ruling out the planet scenario in this system.