We measured the slope of the cool part of the RGB, and obtained . Comparing these values to those given in Ortolani et al. (1991) and Ortolani et al. (1996b), we would assign to Ter 6 a metallicity similar to that of NGC 6553.
An alternative indicator of metallicity is the magnitude difference between the HB level and the top (brightest stars) of the RGB. We find for M30 (Rosino et al. 1996), 2.3 for 47 Tuc, 2.1 for NGC 6356 (Bica et al. 1994), 1.4 for NGC 6528 and 1.3 for NGC 6553 (Ortolani et al. 1995) whereas for Ter 6. This would place Ter 6 with a metallicity intermediate between those of 47 Tuc and NGC 6553/NGC 6528. The cluster is probably more metallic than previously estimated by AZ88.
We calculate the cluster reddening taking NGC 6553 and 47 Tuc as references (Ortolani et al. 1995; Desidera 1996; Bica et al. 1994). The (V-I) colour of the RGB at the HB level for Ter 6 is , while that of NGC 6553 is and for 47 Tuc . The difference between NGC 6553 and Ter 6 is ; given that from HST data the revised value E(V-I) = 0.95 for NGC 6553 (Guarnieri et al. 1996), this leads to . Relative to 47 Tuc, , and given E(V-I) = 0.05 for 47 Tuc, . Adopting the average value , and using equation A1 of Dean et al. (1978) which leads to E(V-I)/E(B-V) = 1.36, we obtain E(B-V) = 2.14 for Ter 6. Assuming (Savage & Mathis 1979) we get ; if R = 3.6 suitable for red stars (Terndrup 1988;; Grebel & Roberts 1995), we get . This difference dominates over all other error sources, amounting to 1.1 magnitudes.
The HB magnitude of Ter 6 is at ; assuming that the absolute magnitude of the HB for a metallicity of about [Fe/H] is 0.92 (Chaboyer et al. 1996), and applying a 0.14 blanketing correction which matches bolometric magnitudes (see discussion in Guarnieri et al. 1996), there results . This value is in good agreement with values given by Buonanno et al. (1989), where however the correction due to the colour difference between the RR Lyraes and our redder stars was not included. We obtain an apparent distance modulus of (m-M) = 21.19. The true distance modulus will depend on the adopted, resulting for R = 3.1 and 13.13 for R = 3.6. This corresponds to a distance of kpc (R = 3.1) and kpc (R = 3.6). These distance values are considerably smaller than the value indicated by Webbink (1985) and more compatible with the value given by Fahlman et al. (1995).
Assuming the standard value of R = 3.1 ( kpc), and assuming the distance of the Sun to the Galaxy center of kpc (Reid 1993), the Galactocentric coordinates are X = 1.0 (X>0 refers to our side of the Galaxy), Y = -0.17 and Z = -0.26 kpc.