  In the orthorhombic manganites o-RMnO3, where R is a heavy rare earth (R =
Gd-Yb), the Mn3+ sublattice is known to undergo two magnetic transitions. The
low temperature phase has an antiferromagnetic structure (collinear or
elliptical), which has been well characterized by neutron diffraction in most
of these compounds. The intermediate phase, occurring in a narrow temperature
range (a few K), is documented for R = Gd-Ho as a collinear modulated
structure, incommensurate with the lattice spacings. We report here on a 57Fe
M\"ossbauer study of 2% 57Fe doped o-YbMnO3, where the spin only Fe3+ ion plays
the role of a magnetic probe. From the analysis of the shape of the magnetic
hyperfine M\"ossbauer spectra, we show that the magnetic structure of the
intermediate phase in o-YbMnO3 (38.0 K < T < 41.5 K) is also modulated and
incommensurate.
