Calibrated colour–magnitude diagrams for Sh2-152 a) and the control field b). The black lines represent isochrones of solar metallicity and 11.2 Gyr, located at three distances (3.0, 4.0 and 5.0 kpc) and the grey curve, the position of a K2 III star at several distances. The isochrone was generated using the Pietrinferni et al. (2004) stellar evolutionary library, the Castelli & Kurucz (2003) bolometric correction library and the Rieke et al. (1989) extinction law with R = 3.09 (Rieke & Lebofsky 1985). For the K2 III sequence, M_V was obtained from Cox (2000) and IR intrinsic colours from Ducati et al. (2001).