Centroid pulse phase averaged cyclotron line energies at different 35 d phases of Main-On cycle 323. The data points with uncertainties are direct measurements for ten small integration intervals. The blue triangles connected by the solid blue line are values which are calculated by folding observed pulse profiles in the 30–45 keV range with template (E_cyc vs. pulse phase)-profiles for the same 35 d phases (see text for a detailed description).