  With Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE)
observations, we detected highly variable helium emission lines from the
optical counterpart of the supersoft ultraluminous X-ray source (ULX) NGC 247
ULX-1. No Balmer lines can be seen in the source spectrum. This is the first
evidence for the presence of a helium donor star in ULXs, consistent with a
prediction that helium donor stars may be popular in ULXs. The helium lines
with a FWHM of about 200 km/s are likely produced on the outer accretion disk.
Their strong variation implies that the central X-ray source can be
significantly obscured to the outer disk. Also, a ring or a double-ring
structure is revealed in the MUSE image. It is unknown whether or not it is
related to the progenitor of the ULX binary.
