  On May 27 2024, the Wide-field X-ray Telescope onboard the Einstein Probe
(EP) mission detected enhanced X-ray emission from a new transient source in
the Small Magellanic Cloud (SMC) during its commissioning phase. Prompt
follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope and
Nicer have revealed a very soft, thermally emitting source (kT$\sim$0.1 keV at
the outburst peak) with an X-ray luminosity of L$\sim$4$\times$10$^{38}$ erg
s$^{-1}$, coincident with CXOU J005245.0-722844. This super-soft outburst faded
very quickly in a week time. Several emission lines and absorption edges were
present in the X-ray spectrum, such as the Oxygen (0.57 keV) and Neon (0.92
keV) He-like emission lines, and deep Nitrogen (0.67 keV) and Oxygen (0.87 keV)
absorption edges. The X-ray emission resembles typical nova outbursts from an
accreting white dwarf (WD) in a binary system, despite the X-ray source being
historically associated with an O9-B0e massive star exhibiting a 17.55 days
periodicity in the optical band. The discovery of this super-soft outburst
nails down CXOU J005245.0-722844 as a BeWD X-ray binary: an elusive
evolutionary stage where two main-sequence massive stars have undergone a
common envelope phase and experienced at least two episodes of mass transfer.
In addition, the very short duration of the outburst and the presence of Ne
features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne-O
WD in the system.
