  The S-index is a measure of emission in the CaII H & K lines and is a widely
used proxy of stellar magnetic activity. It has been assumed until now that the
S-index is mainly affected by bright plage regions in the chromosphere. In
particular, the effect of starspots on the S-index has been neglected. In this
study we revisit this assumption. For this we analyze high-resolution
observations of sunspots recorded in the CaII H spectral line at the Swedish
1-m Solar Telescope and determine the contrast of spots with respect to the
quiet surroundings. We find that the CaII H line core averaged over whole
sunspots (including superpenumbrae) is brighter than in the quiet surroundings
and that the spot contrast in the line core is comparable to the facular
contrast. This allows us to get a first estimate of the influence of spots on
the S-index. We show that spots increase the S-index. While this increase is
quite small for the Sun, it becomes significantly larger for more active stars.
Further, we show that the inclusion of the contribution of spots to the S-index
strongly affects the relationship between the S-index and stellar disk area
coverages by spots and faculae, and present the new relations.
