LAMOST J040901.83+329355.6 - a new Galactic star with Wolf-Rayet characteristics in the transitional stage from post-asymptotic giant branch to central star of a planetary nebula

The similarity in physical conditions in the winds of low-mass post-asymptotic giant branch stars and evolved massive stars leads to the appearance of an interesting phenomenon of spectral mimicry. Because of this, the discovery of every new star with a Wolf-Rayet (WR) spectrum requires a special study of its evolutionary status before it can be included in the list of Galactic WR stars. A couple of years ago, LAMOST J040901.83+323955.6 (hereafter J0409+3239) was selected as a WR star in the LAMOST spectroscopic data base by machine-learning methods. In this work, we investigate its evolutionary status. After analysing the spatial location of J0409+3239 in the Galaxy and its position in the colour-magnitude diagram, we conclude that J0409+3239 is instead a low-mass object displayng the WR phenomenon. Its luminosity is $L*=1000~\rm L_\odot$ and its effective temperature is T_eff = 40 000 K. Using new and archival photometric data, we detected irregular variability on time-scales from hours to tens of days with an amplitude of up to ~0.2 mag. A comparison of the spectrum obtained in 2022 with that from 2014 also shows evidence of spectral variability. The absence of a clearly detected circumstellar nebula prevents the classification of J0409+3239 as [WR], namely as the central star of a planetary nebula (CSPN). However, the position of J0409+3239 on the Hertzsprung-Russell diagram suggests that this object is a low-mass star caught in a rare transitional phase to CSPN. Estimation of the J0409+3239 mass based on evolutionary tracks shows that it is less than $0.9~\rm M_\odot$, and thus that the age of the Galaxy is barely sufficient for the star to have evolved to its current stage.