Central stars of newly discovered infrared nebulae: eruptive Be stars PY Gem and HD 253659

Abstract The presence of a nebula around massive hot stars often works as an indicator that the object is either in an advanced evolutionary stage or a rare product of close binary interaction. Here, we focus on two Be stars – PY Gem and HD 253659 – whose nebulae were detected with the Wide-field Infrared Survey Explorer. We estimate their basic physical parameters from modelling the spectral energy distribution and examine their variability. We combine archival photometric data from several ground-based survey telescopes and from the Transiting Exoplanet Survey Satellite (TESS) with our own data from dedicated multi-colour photometric and spectroscopic monitoring, and conclude that, despite having similar stellar properties, the photometric variability of the two objects is strikingly different. For HD 253659 we detect continuous eruptive variability and a major outburst that took place between 2016 and 2022 along with numerous flicker events seen in the TESS data. HD 253659 also exhibits loop-like behaviour in the colour–magnitude diagram, consistent with build-up and dissipation phases of a circumstellar disk seen close to pole-on. In contrast, during the past ~20 years PY Gem has lost its eruptive variability and shows only small-amplitude p and g-mode pulsations classifying PY Gem as β Cephei hybrid pulsator. An incoherent low-frequency signal is identified as a Štefl frequency, which seems to be supported by the cyclic variation of the emission-line profiles. Detailed analysis of these stars indicates that both objects are inconsistent with evolved massive stars, and that the origin of their nebulae is more likely linked to the physics of the Be phenomenon.