Dip spectrum, shown with dotted blue lines, most likely represents emission that is also present during the bright spectrum (red dotted lines). It probably does not originate from the SSS photosphere but further outside, likely the shock emission site. As an illustration of how the pure SSS spectrum may look, the difference between bright and dip spectrum is shown with black solid lines for oxygen (left) and the nitrogen lines (right; see labels). See Sect. 3.2.3 for discussion.