  With the present and soon-to-be breakthrough of optical interferometry,
countless objects shall be within reach of interferometers; yet, most of them
are expected to remain only marginally resolved with hectometric baselines.
  In this paper, we tackle the problem of deriving the properties of a
marginally resolved object from its optical visibilities. We show that they
depend on the moments of flux distribution of the object: centre, mean angular
size, asymmetry, and curtosis. We also point out that the visibility amplitude
is a second-order phenomenon, whereas the phase is a combination of a
first-order term, giving the location of the photocentre, and a third-order
term, more difficult to detect than the visibility amplitude, giving an
asymmetry coefficient of the object. We then demonstrate that optical
visibilities are not a good model constraint while the object stays marginally
resolved, unless observations are carried out at different wavelengths.
Finally, we show an application of this formalism to circumstellar discs.
