2007MNRAS.379..159C

New CCD photometry in the Washington system C and T₁ passbands down to T₁∼ 18.5 mag in the field of the northern open cluster NGC2236 is presented. T₁ magnitudes and C - T₁ colours for a total of 1162 stars within an area of 13.6x13.6 arcmin² were measured. These CCD data were supplemented with photoelectric CMT₁T₂ photometry of 13 red giant candidates. The comparison of the cluster (T₁, C - T₁) colour-magnitude diagram with theoretical isochrones computed for the Washington system yields E(C - T₁) = 1.10±0.10 and for log t = 8.80 (t = 600⁺¹⁰⁰_–40 Myr) and Z = 0.008. The derived E(C - T₁) value implies E(B - V) = 0.55±0.05. NGC2236 is then located at 2.5±0.5 kpc from the Sun beyond the Perseus spiral arm and at ∼10.8 kpc from the Galactic centre. A cluster angular diameter of 9.4 arcmin, equivalent to 6.8 pc, was estimated from star counts both within and outside the cluster field. We also derived from the stellar density profile a cluster core radius of r_c= 1.7 arcmin (1.2 pc) and an annular corona of Δr_c= 1.8r_c(2.2 pc). Five independent Washington abundance indices yield a mean cluster metallicity of [Fe/H] = -0.3±0.2, which is not only in reasonably good agreement with the one obtained from the isochrone fit, but also compatible with the existence of a radial abundance gradient in the Galactic disc. We examined the properties of a sample of 20 known open clusters aligned along the line-of-sight to NGC2236. Berkeley27 appears as the farthest and oldest cluster of the studied sample.