IC 1613’s U − B vs. Q diagram. Black dots mark catalogue stars with high-quality photometry from GHV09. Other colours and symbols, except for filled stars, mark the position of stars with known spectral types from Bresolin et al. (2007). O- and early-B-stars are concentrated in boxes 1 and 2. The pink star represents 62 024, an Of star analysed by Herrero et al. (2012). Red stars represent the sample of Tramper et al. (2011) in IC 1613. Green stars mark this paper’s targets.