Discovery of a probable very fast extragalactic nova in a symbiotic binary

Very fast novae are novae that evolve exceptionally quickly (on timescales of only days). Due to their rapid evolution, very fast novae are challenging to detect and study, especially at early times. Here we report the discovery, made as part of our Transient UV Objects project, of a probable very fast nova in the nearby spiral galaxy NGC 300. We detected the rise to the peak (which are rarely observed for very fast novae) in the near-ultraviolet (NUV), with the first detection just ~2 h after the eruption started. The peak and early stages of the decay were also observed in UV and optical bands. The source rapidly decayed (two NUV magnitudes within 3.5 days), making it one of the fastest novae known. In addition, a likely quiescent counterpart was found in archival near-infrared Spitzer and VIRCAM images, but not in any deep optical and UV observations, indicating a very red spectral shape in quiescence. The outburst and quiescence properties suggest that the system is likely a symbiotic binary. We discuss this new transient in the context of very fast novae in general and specifically as a promising supernova Type Ia progenitor candidate, due to its very high inferred WD mass (~1.35 M_⊙; determined by comparing this source to other very fast novae).