  Magnetically-driven ferroelectric atomic displacements of the order of
10$^{-3} Angstrom have been observed in orthorhombic (perovskite like) YMnO$_3$
by a single-crystal synchrotron x-ray diffraction. The refined polar structure
shows the characteristic bond alternation driven by the exchange striction in
staggered Mn-O-Mn arrays with $\uparrow\uparrow\downarrow\downarrow$ type
ordering, giving rise to a spontaneous polarization along a-axis.
First-principles calculations based on the Berry phase method as well as on the
experimentally refined crystal structure can reproduce the observed
polarization value.
