<qa>  What is the primary objective of the study?The main objective of the study is to evaluate the additional effect of a narrow band Type B Ultraviolet UVB therapy on the treatment of severe plaque psoriasis with fumaric acid esters FAE in comparison to FAE monotherapy. The secondary objectives are to evaluate the effect of an additional narrow band UVB therapy on the cumulative FAE dose required to reach Psoriasis Area and Severity Index PASI 75 in comparison to FAE monotherapy and to evaluate whether a leukopenia and lymphopenia frequently occurring during the FAE treatment is a positive predictive factor for the treatment response. 