Stark-Wannier type operators with purely singular spectrum

We consider the one-dimensional Stark-Wannier type operators with potentials given by a smooth function with a logarithmic growth at infinity plus a periodic function with the Fourier coefficients of the form $(\ln |n|)^{-b}, 0<b<1/2$. We prove that in the case of rational electric field the spectrum of the corresponding operator is purely singular continuous.