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We investigate the nature of CXOU J005440.5-374320 (J0054), a peculiar bright($\sim$$4\times10^{39}$ erg/s) and soft X-ray transient in the spiral galaxyNGC 300 with a 6-hour periodic flux modulation that was detected in a 2014Chandra observation. Subsequent observations with Chandra and XMM-Newton, aswell as a large observational campaign of NGC 300 and its sources performedwith the Swift Neil Gehrels Observatory, showed that this source exhibitsrecurrent flaring activity: four other outbursts were detected across $\sim$8years of monitoring. Using data from the Swift/UVOT archive and from theXMM-Newton/OM and Gaia catalogues, we noted the source is likely associatedwith a bright blue optical/ultraviolet counterpart. This prompted us to performfollow-up observations with the Southern African Large Telescope in December2019. With the multi-wavelength information at hand, we discuss severalpossibilities for the nature of J0054. Although none is able to account for thefull range of the observed peculiar features, we found that the two mostpromising scenarios are a stellar-mass compact object in a binary system with aWolf$-$Rayet star companion, or the recurrent tidal stripping of a stellarobject trapped in a system with an intermediate-mass ($\sim1000$ $M_\odot$)black hole.

A soft and transient ultraluminous X-ray source with 6-h modulation in the NGC 300 galaxy