The evolution of asymptotic giant branch stars from the spherical symmetry into the diverse shapes of planetary nebulae (PNe) is a topic of intensive research. Young PNe provide a unique opportunity to characterize the onset of this transitional phase. In particular, OH maser-emitting PNe (OHPNe) are considered nascent PNe. In fact, only six OHPNe have been confirmed to date. In order to identify and characterize more OHPNe, we processed the unpublished continuum data of the interferometric follow-up of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). We then matched the interferometric positions of OH maser and radio continuum emission, considering the latter as a possible tracer of free–free emission from photoionized gas, characteristic of PNe. We report eight objects with a positive coincidence, four of which are classified as candidate OHPNe here for the first time (IRAS 16372–4808, IRAS 17494–2645, IRAS 18019–2216, and OH 341.6811+00.2634). Available evidence strongly indicates that they are evolved stars, while the comparison with confirmed OHPNe indicates that they are likely to be PNe. Their final confirmation as bona fide PNe, however, requires optical/infrared spectroscopy. The obtained spectral indices of the radio continuum emission (between ≃0.4–1.3) are consistent with partially optically thick free–free emission from photoionized gas. Also, they cluster in the same region of a WISE colour–colour diagram as that of the confirmed OHPNe (⁠|$9.5\lesssim[3.4]{-}[22]\lesssim13.5$|⁠, and |$4.0\lesssim[4.6]{-}[12] \lesssim7.0$|⁠), thus this diagram could help to identify more OHPNe candidates in the future.

Stars such as the Sun expel their outer layers and form planetary nebulae (PNe) as they evolve into white dwarfs. PNe exhibit diverse morphologies, the origins of which are not fully understood. PNe with OH (OHPNe) and H ₂ O (H ₂ OPNe) masers are thought to be nascent PNe. However, the number of known OHPNe and H ₂ OPNe remains small, and only in eight cases has the position of the maser emission been found to coincide with the PN, using the high astrometric accuracy of interferometric observations. In order to identify more OHPNe and H ₂ OPNe, we used public databases and our own ATCA/VLA observations to match the positions of OH and H ₂ O masers with known PNe and radio continuum emitters, considering radio continuum emission as a possible tracer of the photoionized gas that characterizes PNe. Here we report the confirmation of positional coincidence of maser emission with one more PN, and 12 PN candidates. Moreover, we have confirmed three evolved stars as ‘water fountains’ (WFs) hosting H ₂ O masers. These WFs are associated with radio continuum emission, but their possible nature as PNe has not yet been confirmed. Although a final characterization of maser-emitting PNe as a group still requires confirmation of more objects, their distribution in the infrared color-color diagrams suggests that they are a heterogeneous group of PNe. In particular, the new OHPN IRAS 07027–7934 has been reported to contain a late [WC]-type central star, while the maser emission implies an O-rich envelope. This property is found in only one other known maser-emitting PN, although we found evidence that other confirmed and candidate OHPNe may also have mixed chemistry, since they show emission from polycyclic aromatic hydrocarbons. The new WF IRAS 18443–0231 shows radio continuum that is dominated by strong and variable non-thermal emission, as in magnetized outflows.

IRAS 22568+6141 has been classified as a low-excitation planetary nebula (PN) and presents non-thermal radio continuum emission, which could be a signature of a nascent PNe. We present intermediate-resolution long-slit spectra obtained in 2021 and 2023, high-resolution long-slit spectra taken in 2023, and a light curve at the r filter between 1953 and 2019. They all reveal changes in IRAS 22568+6141 with timescales of decades and a few years. The object underwent an energetic event around 1990 that suddenly increased its brightness, which has been fading since then. A comparison with a published spectrum from 1988 shows an increase in the Hβ flux in 2021 by factor of ≃6 and the [O III] emission lines, which were absent in 1988. Between 2021 and 2023, the Hβ flux decreased by a factor of ≃1.7, and the [O III] emission lines almost vanished. These results and the variability observed in other emission lines indicate that IRAS 22568+6141 was recombining and cooling between 2021 and 2023, and probably since 2005, as suggested by archival radio continuum and mid-IR observations. The intermediate- and high-resolution spectra show that the excitation of the emission lines was dominated by shocks in 2021 and 2023, and probably also in 1988, which may be related to the non-thermal radio continuum emission from the object. Although the variability might be due to changes in the physical conditions in the shocks or in a nova-like eruption, it better accommodates that expected from a late thermal pulse, which is further suggested by a comparison with other similar objects. New observations and monitoring in the coming years are crucial to corroborate the origin of the variability.

Planetary nebulae (PNe) harbouring masers of H₂O (H₂OPNe) and/or OH (OHPNe) are thought to be nascent PNe. They are extremely scarce, and so far only eight members are know to date. Here we explain our current effort to identify new H₂OPNe and/or OHPNe. We report IRAS 07027–7934 as a new bona fine OHPN. Its 1612 MHz OH spectrum seems to be changing from double- to single-peaked since the redshifted emission has vanished almost completely, and the 1667 MHz OH maser emission has disappeared. For the OHPN Vy 2-2, we found that its central star is unexpectedly carbon (C)-rich, has a low-mass progenitor, and could be a post-common envelope binary system. Moreover, we confirm Vy 2-2 as a nascent PN. We speculate that low-mass C-rich central stars in post-common envelope systems could be a common end of H₂OPNe and OHPNe.

Observations in the millimetre bands of maser-emitting planetary nebulae (PNe) are crucial to study their circumstellar molecular gas at the beginning of the PN phase. Maser-emitting PNe are in the earliest phases of PN formation; therefore, these sources are key objects to study the molecular content during the early evolution of PNe. These circumstellar envelopes are active sites for the formation of molecules. We present preliminary results of millimetre observations with the IRAM 30 m telescope towards one PN (IRAS 17393−2727) of a sample of five maser-emitting PNe, where we detect ${}^{12}$CO and ${}^{13}$CO lines in both $J=1\to 0$ and $J=2\to 1$ transitions.

OH masers are signpost of planetary nebulae (PNe) at a very young stage. In particular, Vy 2-2 was the first OH maser-emitting PN ever identified. It consists of a bright compact shell and a faint bipolar structure. The reported values for its O⁺²/H⁺ abundance discrepancy factor (ADF) are inconsistent:; 4.3, and; 11.8. To characterize the ionized gas and to redetermine the ADF(O⁺²), we have obtained medium-resolution optical spectra. We found that the ionized gas is oxygen-rich (C/O; 0.15), and an ADF(O⁺²); 13.6. Also, the N/O abundance ratio is; 0.39, implying a low-mass progenitor (M_i ≤ 1.5 M_e). Unexpectedly, the detection of stellar weak emission lines (wels) suggests that the central star is carbon-rich. Alternatively, the wels may be emitted by an irradiated companion in a post-common envelope system. We have also obtained high-resolution optical spectroscopy on Vy 2-2. Our spectra kinematically resolve the bright shell and bipolar structure. We found that these expand at: 10 and: 20 km s^-1, respectively.