J–H versus H–K color–color diagram from the UKIDSSDR8PLUS database. Black squares are B- and A-type stars, red squares are cooler F- and G-type stars. The open gray squares are unclassified stars. The theoretical stellar evolution isochrone for 5 Myr from the Geneva stellar models is shown (Ekström et al. 2012; Yusof et al. 2013). MWC 137, #10, #23, #25, and #28 are saturated in the UKIDSS images, and their JHK values were obtained from the 2MASS point source catalog (Cutri et al. 2003; the 2MASS catalog does not contain the star #10, which is thus not displayed here). See text for details.