Top panel: spectrum of NGC 1806 with overplotted central passband (red) and the classical continuum region (blue) for the C₂ index (Maraston 2005). In orange we overplot the new continuum region for the C₂ index defined in this paper (see Sect. 4). Bottom panel: comparison between the classical and the redefined C₂ index, measured on our own LMC star observations (solid black and red symbols), as well as the stars from the Milky Way spectral libraries of Lançon & Mouhcine (2002, orange) and Rayner et al. (2009, black diamonds, green squares, red asterisks). The dashed line shows the one-to-one relation, the solid line represents a linear least squares fit to all data points.