f_{24 μm}/f_R as a function of R − K colour for all the COSMOS 24 μm sources associated with the optical and K-band counterparts (green isodensity contours). Black crosses represent the 1 mJy flux-limited sample. Yellow stars denote the five objects observed with SINFONI; cyan stars and squares denote Melbourne et al. (2011) and Brand et al. (2007) targets, respectively (two targets present in both samples are indicated with stars); blue symbols denote the Perna et al. (2015) targets. Star and square symbols indicate IFU and long-slit observations, respectively. Sources marked with red circles are also detected in the 1 mJy flux limited sample and are those considered for the analysis. The box delimited by a vertical solid line and horizontal dashed line marks the region proposed in Fiore et al. (2009) to select CT AGN (Cell A). The horizontal dashed line sets the criterion for the selection of DOGS.