Left: K_s versus (J − K_s) CMD for the circular region around the window as shown in Fig. 1. The blue region was used to derive the RC mags and colours. The contours trace iso density levels. A reddening vector with the slope of A_Ks = 0.484 × E(J − K_s) is also shown. Right: normalised density distribution of selected RC versus distance moduli of the low extinction window (blue squares) and of the comparison field (green squares).