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Some transients, although classified as novae based on their maximum andearly decline optical spectra, cast doubts on their true nature and whethernova impostors might exist. We monitored a candidate nova which displayed adistinctly unusual light curve at maximum and early decline through opticalspectroscopy (3000-10000 \AA, 500<R<1) complemented with Swift UV andAAVSO optical photometry. We use the spectral line series to characterize theejecta dynamics, structure, and mass. We found that the ejecta are in freeballistic expansion and structured as typical of classical novae. However,their derived mass is at least an order of magnitude larger than the typicalejecta masses obtained for classical novae. Specifically, we foundM$_{ej}\simeq$9$\times$10$^{-3}$ M$_\odot$ independent of the distance for afilling factor $\varepsilon$=1. By constraining the distance we derived$\varepsilon$ in the range 0.08-0.10, giving a mass 7$\times$10$^{-4}\lesssim$M$_{ej}\lesssim$9$\times$10$^{-4}$ M$_\odot$. The nebular spectrum,characterized by unusually strong coronal emission lines, confines the ionizingsource energy to the range 20-250 eV, possibly peaking in the range 75-100 or75-150 eV. We link this source to other slow novae which showed similarbehavior and suggest that they might form a distinct physical sub-group. Theymay result from a classical nova explosion occurring on a very low mass whitedwarf or be impostors for an entirely different type of transient.

The ambiguous transient ASASSN-17hx. A possible nova-impostor