VFTS 711, 080, and 109 are examples of cases for which only upper limits could be derived. Left panels: the observed spectrum is shown in blue, the region used to determine the S/N in red, and the model representing the upper limit in black. Middle panels: zoom in on the fitted region. For these cases this region is used to determine the S/N. The red line is the mean flux level of this range; the dashed lines indicate the standard deviation. Right panels: theoretical depth values as a function of ε_N in black, where the minimal detectable depth and its inferred upper limit are indicated as the red horizontal and the blue vertical line respectively. The resulting upper limit is indicated in the top left corner of the right panel. The line depression for the upper limit to ε_N can be considerably larger than the noise would suggest as a result of our conservative choice for this limit (see Sect. 3.3).