(Left) (J − H)–(H − K) colour–colour diagram of sources within 50′′ of IRAS 18511+0146 (black dots). These sources and the associated JHK_S magnitudes are found in the 2MASS All-Sky Point Source Catalogue. The red dashed lines delimit the reddening band of main sequence stars. The effect of an interstellar extinction of A_V = 5 is indicated by the red arrow (Rieke & Lebofsky 1985). The loci of main sequence and giant branch dereddened stars are represented by the green crosses (Bessell & Brett 1988) and the corresponding stars reddened by A_V = 5 are the red crosses. (Right) Same as the left diagram, but with sources lying within a radius of 150′′ of IRAS 18511+0146. A visual extinction vector of A_V = 18 and the corresponding reddened main sequence stars are represented in blue. The positions of S7, S10, and S11 in this diagram are indicated, but for S10 and S11 only lower limits are given because of the non-detection of these source in the J band (cf. Table 2).