  We have identified, for the first time, the change in symmetry at the
ferroelectric transition TFE near 1023K of the ferroelectromagnet YMnO3. This
transition takes place 300K below the transition to the centrosymmetric state
at TIP. Single crystal synchrotron diffraction coupled to a group theoretical
analysis show that the paraelectric intermediate phase between TIP and TFE has
P63/mcm symmetry. This proves that YMnO3 is a proper ferroelectric and not an
improper ferroelectric, as suggested by a previous group theoretical
assignment. The origin of the ferroelectricity is caused by a correlated
tilting of the MnO5 polyhedra along the (100), (110) and (010) directions
