Ultra-long-period Cepheids as Standard Candles from Gaia to Rubin-LSST

This work presents an analysis of the largest known sample of 73 Ultra Long Period Cepheids (ULPs), including 15 already known objects in nearby galaxies, with new accurate and homogeneous photometry by Gaia, and a new object, in our Galaxy, identified as Long Period Variable in Gaia, but recently reclassified as ULP. The obtained results suggest that improving the photometric accuracy, the ULP Period-Wesenheit relation has a smaller dispersion and a better agreement with the Classical Cepheid (CC) one, supporting the hypothesis that ULPs are the extension of CCs at longer periods and higher luminosity, even if some of their properties represent a challenge for the evolutionary and pulsational models in particular in the very metal poor regime. If these highly luminous variables are shown to be standard candles, they can directly probe cosmological distances without any secondary distance indicator and help understand the well known Hubble tension. In this framework, the potential of the Rubin-LSST survey to enlarge the sample of ULPs with new multi-filter homogeneous data has also been explored.