  The superconductor-ferromagnet-superconductor $\varphi_{0}$ junction provides
a direct coupling between Josephson phase and magnetic moment of ferromagnetic
barrier. We demonstrate an appearance of additional fractional subharmonic
steps in the IV-characteristics of $\varphi_{0}$ junction under external
electromagnetic radiation due to spin-orbit coupling. An origin of subharmonic
steps is related to the locking of magnetic moment precession to the Josephson
oscillations. We prove that the positions of those steps follow a continued
fraction algorithm.
