  The upcoming extremely large telescopes will have to deal with the so-called
''pupil fragmentation'' effect: for what concerns the Extremely Large Telescope
(ELT), the presence of thick spider legs supporting the secondary mirror may
induce unseen phase discontinuities across the pupil sectors that could limit
the performance of the adaptive optics correction. In this context, we propose
a wavefront sensor (WFS), the CiaoCiao WFS, consisting in a rotational shearing
interferometer to sense phase differences between the pupil sectors. In this
work, we present the CiaoCiao WFS concept and the first analyses carried out
through numerical simulations. In particular, we analyze the performance of
such a wavefront sensor in case the phase discontinuities are induced by
low-wind effect during observations with the Multiconjugate adaptive Optics
Relay For ELT Observations (MORFEO).
