Alternative arrangement for the emission regions of the Si XIV, S XVI, Ar XVIII, and Ca XX lines. Parts of the Ca and Ar lines formed in the compact star disc or corona (see the text). The solid curves represent surfaces with a constant ionisation parameter marked by a log ξ-value. These curves are intersections with the (x,y) plane of 3D surfaces where the lines are expected to form. For clarity, the error bars have been shifted vertically from y=0. The coordinate system is the same as in Fig. 4.