  The experimental observation of multiferroic behavior in perovskite
manganites with a spiral spin structure demands to clarify the origin of these
magnetic states and their relation to ferroelectricity. We show that
spin-spiral phases with diagonal wavevector and also the E-type phase exist for
intermediate values of the Hund's rule and the Jahn-Teller coupling in the
orbitally ordered and insulating state of the standard two-band model
Hamiltonian for manganites. Our results support the spin-current mechanism for
ferroelectricity and present an alternative view to earlier conclusions where
frustrating superexchange couplings were crucial to obtain spin-spiral states.
