  Context. During four all-sky surveys (eRASS1--4), eROSITA, the soft X-ray
instrument aboard Spektrum-Roentgen-Gamma (SRG) detected a new supersoft X-ray
source, eRASSU J060839.5-704014, in the direction of the Large Magellanic Cloud
(LMC). Methods. We arranged follow-up observations in the X-ray and optical
wavelengths and further searched in archival observations to reveal the nature
of the object. Results. We discover pulsations at ~374 s with a pulse profile
consistent with 100% modulation. We identify two other periodicities in the
eROSITA data, which we establish as aliases due to the sampling of the eROSITA
light curve. We identify a multi-wavelength counterpart to the X-ray source in
UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton
and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The
timing and spectral characteristics of the source are consistent with a double
degenerate ultra-compact binary system in the foreground of the LMC. eRASSU
J060839.5-704014 belongs to a rare class of AM CVns, which are important to
study in the context of progenitors of SN Ia and for persistent gravitational
wave detection. Conclusions. We identify eRASSU J060839.5-704014 as a new
double degenerate ultra-compact binary located in the foreground of the LMC.
