  We consider quasi one-dimensional RKKY systems in proximity to an s-wave
superconductor. We show that a $2k_F$-peak in the spin susceptibility of the
superconductor in the one-dimensional limit supports helical order of localized
magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The
magnetic helix is equivalent to a uniform magnetic field and very strong
spin-orbit interaction (SOI) with an effective SOI length $1/2k_F$. We find the
conditions to establish such a magnetic state in atomic chains and
semiconducting nanowires with magnetic atoms or nuclear spins. Generically,
these systems are in a topological phase with Majorana fermions. The inherent
self-tuning of the helix to $2k_F$ eliminates the need to tune the chemical
potential.
