  Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can
switch from a supersoft spectral state, where most of the luminosity is emitted
below 1 keV, to a soft spectral state with significant emission above 1 keV. In
a few systems, dips have been observed. The mechanism behind this state
transition and the dips nature are still debated. To investigate these issues,
we obtained a long XMM-Newton monitoring campaign of a member of this class,
NGC 247 ULX-1. We computed the hardness-intensity diagram for the whole dataset
and identified two different branches: the normal branch and the dipping
branch, which we study with four and three hardness-intensity resolved spectra,
respectively. All seven spectra are well described by two thermal components: a
colder ($kT_{\rm bb}$ $\sim$ 0.1-0.2 keV) black-body, interpreted as emission
from the photo-sphere of a radiatively-driven wind, and a hotter ($kT_{\rm
disk}$ $\sim$ 0.6 keV) multicolour disk black-body, likely due to reprocessing
of radiation emitted from the innermost regions. In addition, a complex pattern
of emission and absorption lines has been taken into account based on previous
high-resolution spectroscopic results. We studied the evolution of spectral
parameters and the flux of the two thermal components along the two branches
and discuss two scenarios possibly connecting the state transition and the
dipping phenomenon. One is based on geometrical occultation of the emitting
regions, the other invokes the onset of a propeller effect.
