Very metal-poor massive stars hold the key to interpret high-redshift star-forming galaxies and the early reionization epoch, but also contemporary events such as gravitational waves. To study these objects in resolved environments, we need to resort to dwarf irregular galaxies far from the potential wells of M31 and the Milky Way, and therefore distant. While the archives, recently boosted by the ULLYSES and XSHOOTU programs, store a healthy dataset of massive stars in the Milky Way and the Magellanic Clouds, the number of observed targets with poorer metal content than the SMC (1/5 Zȯ) is dramatically small. This paper reviews the state of observations of very metal-poor massive stars, assessing what can be realistically learned about their physics and evolution with current instrumentation, and arguing whether or not near-future facilities can remedy the gaps in the knowledge that remain.

The results of long term observations of photometric ($UBV$) and spectral variability of several PNe are presented. For the most variable nebulae, IC 4997 and NGC 6572, showing both photometric and spectral changes in time, the main physical parameters ($T_{e}$, $N_{e}$ and $T_{*}$) and their time variations were estimated and discussed.