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tokens: This is well in the range of those measured for UCXBs. 4.2. The nature of the accretor Determining the nature of the compact object in an unclassified LMXB is often challenging. It can be assessed based either on a dynamical measurement of the mass of the compact object (via optical and near infrared spectroscopy during quiescence; see e.g. Casares et al. 1992 ), or on the detection of X-ray coherent pulsations and/or thermonuclear X-ray bursts, which would provide a straightforward observational evidence for a NS accretor. To-date, all confirmed UCXBs are known to contain a NS accretor, although there are a number of BH candidates (see e.g. Bahramian et al. 2017 ). Assuming that our identification of J0840 as a UCXB is correct, it is then tempting to conclude that this system hosts a NS as well. However, our non-detection of coherent pulsations or thermonuclear X-ray bursts leaves this as an open question. 4.2.1. On the lack of coherent X-ray pulsations Coherent pulsations are thought to be formed in NS systems where the magnetic field is large enough to truncate the accretion disk and channel part of the disk material onto a small region on the NS surface, close to the magnetic poles. The radiation emitted from the heated impact region (hot spot) or from a slab of shocked plasma that forms above it (accretion columns) appears modulated at the NS spin period, producing a pulsed emission component. About 30 accreting millisecond X-ray pulsars have been detected to date (for reviews, see Di Salvo Sanna 2020 ; Patruno et al. 2021 ) and some of them were seen to exhibit pulsed fractions as small as a few percent in outburst (e.g. Strohmayer et al. 2018 ), that is, below the upper limits estimated for J0840 (Sect. 3.2 ). Besides, three accreting NS–LMXBs have shown coherent X-ray pulsations only sporadically during their outbursts, with the most extreme case being represented by Aql X-1, which showed pulsations over a ∼150 s interval out of a total observing time of ∼1.5 Ms ( Casella et al. 2008 ). However, most NS–LMXBs are non-pulsating systems, and a systematic analysis by Patruno et al. (2018) suggested that weak pulsations might not form at all in (most) non-pulsating LMXBs. Our non-detection of pulsations from J0840 is thus not sufficient to conclusively rule out a NS accretor and might still point either to an accreting millisecond pulsar characterised by a very small X-ray pulsed fraction and/or intermittent pulsations, or a non-pulsating NS–LMXB. 4.2.2. On the lack of thermonuclear X-ray bursts Many NS–LMXBs exhibit thermonuclear (type-I) X-ray bursts due to unstable ignition of hydrogen and/or helium freshly accreted onto the NS surface (for a review, see Degenaar et al. 2018 ). Bursts triggering all-sky monitors (including Swift /BAT) have been detected from several NS–LMXBs and in some cases they even led to the discovery of new transient systems (e.g. Wijnands et al. 2009 ; Degenaar et al. 2012 ). These episodes typically last a few