Interferometry in the visible now provides milliarcsecond spatial
resolution and thus has been intensely used for studying the
circumstellar environment of evolved and pre-sequence stars. Up to
now, most of the scientific results were obtained using only the
modulus of the visibility as a function of baseline, time or projected
baseline (the so-called "super-synthesis" effect). In this paper we
will try to show that even without images, the phase of the visibility
is able to put very strong constraints on stellar modeling since it is
possible to achieve μarsecond measurements using this technique.
We will illustrate through two examples from the GI2T interferometer
how the modulus and the phase of the visibility were used to
understand the physics of Be disks. We will stress on the importance
and the potential of coupling high angular resolution with high
spectral resolution for the study of stellar physics. Finally, We
will present a possible study on the phase signatures due to stellar
rapid rotation that can be done using the VLTI interferometer.