The globular cluster Terzan 6 was discovered by Terzan (1968) with Schmidt plates obtained at the Haute-Provence Observatory. The cluster is also known as GCL B1747-3115, HP5, BH249 and ESO455-SC49. The coordinates are , 15' 44'', and it is located at , .
By means of the bright giants method, Webbink (1985) estimated an horizontal branch level of , and a reddening of E(B-V) = 1.46 from the modified cosecant law, deriving a distance from the Sun of d = 12.8 Kpc. Using integrated near-infrared spectroscopy, Armandroff & Zinn (1988) derived [Fe/H] = -0.61 from CaII triplet lines, and E(B-V) = 2.93 from the interstellar band at 8621 .
Fahlman et al. (1995) presented JHK photometry of Terzan 6, deriving a reddening E(B-V) = 2.04, distance modulus (m-M) = 20.53 and distance to the Sun kpc, and they conclude that the metallicity would be similar to that of M71 ([Fe/H] , Zinn & West, 1984). The cluster structure is very concentrated, with c = 2.50, and it presents a post-core-collapse morphology (Trager et al. 1995). An X-ray source is present (Predehl et al. 1992).
In the present paper we analyse CMDs of Ter 6 in the V, I and Gunn z passbands. In Sect. 2 we describe the observations. In Sect. 3 we present the colour-magnitude diagrams. In Sect. 4 the cluster parameters are measured. The conclusions of this work are given in Sect. 5.