  We discuss the subject of Unextendible Product Bases with the orthogonality
condition dropped and we prove that the lowest rank non-separable
positive-partial-transpose states, i.e. states of rank 4 in 3 x 3 systems are
always locally equivalent to a projection onto the orthogonal complement of a
linear subspace spanned by an orthogonal Unextendible Product Basis. The
product vectors in the kernels of the states belong to a non-zero measure
subset of all general Unextendible Product Bases, nevertheless they can always
be locally transformed to the orthogonal form. This fully confirms the
surprising numerical results recently reported by Leinaas et al. Parts of the
paper rely heavily on the use of Bezout's Theorem from algebraic geometry.
