4.5 Line depth ratios
Spots on slowly rotating stars can be studied by photometric and spectroscopic molecular line
techniques. To discriminate between spot temperature and spot area, these techniques use at least two
independent measurements, brightness and colour in photometry and different molecular bands in
spectroscopy. The Line Depth Ratio (LDR) technique offers an opportunity to resolve temperature
variations with unprecedented accuracy of less than 
(Gray, 1996). A technique for studying
starspots with LDR was developed by Catalano et al. (2002
). It is based on the analysis of depth ratios of a
pair of lines, one insensitive and one very sensitive to temperature. Due to presence of a dark spot that
produces a decrease of say 10% in the continuum, the depth of both lines would be equally
affected and reduced by 10%, while their depth ratio would remain unchanged. Due to the lower
temperature of the spot, however, the temperature sensitive line will change its intrinsic depth and,
thus, the depth ratio of the two lines will change as well. The amount of depth ratio variation
depends on the temperature variation and the fraction of surface covered by spots. Catalano
et al. (2002) proposed 10 pairs of lines in the 
wavelength range and calibrated them
against effective temperatures of main-sequence stars and giants. Their results for active stars are
discussed in Section 5.1. Frasca et al. (2005) combined the LDR technique with the analysis
of simultaneous light curves. This technique enables disentangling the spot temperature and
area.
