The concept of nematic phase was introduced about 20 years ago in order to explain some unusual properties observed in the some unconventional superconducting (SC) systems. Our aim is to carry out a very accurate structural analysis at low T of pure and Mn-substituted FeSe (the simplest among the Fe-based SC) in order to understand if nematicity drives the structural transition or rather a charge density wave (CDW) instability. In fact, we recently revealed how Mn-substitution is able to pin a CDW in Fe-based SC, adding a new fundamental ingredient to explain their physics. By means of Mn substitution, we are confident to reveal if an incommensurate modulation (marking a CDW state) characterizes also the crystal structure of FeSe. The aim of the present proposal is to determine the occurrence of a possible incommensurate structural modulation in pure and very lightly Mn-substituted FeSe .