We show in Fig. 4 (click here)
the V vs. (V-I) diagram for the Danish whole frame
excluding the cluster (r > 44'').
Two striking sequences are seen: the disk main sequence (MS)
and a very extended red horizontal branch (HB) of the bulge
population. The extension of the bulge HB is due to a
differential reddening as large as 
.
A few bulge giants are seen attaining very red colours.
An extraction of the cluster for r < 10'', using the NTT data,
is given in Fig. 5 (click here). The cluster red giant branch (RGB)
can be recognized but not the HB. The brightest giants (BG)
are located at 
and
.
Figure 4: Terzan 10: V vs. (V-I)
colour-magnitude diagram for the Danish whole frame
excluding the cluster (r > 44'')
Figure 5: Terzan 10: V vs. (V-I)
colour-magnitude diagram using the NTT
data, for a circular extraction of radius r < 10''
The RGB sequence is wide and not much populated, so that
it is difficult to estimate its metallicity from Fig. 5 (click here).
It could be fitted by the CMD mean loci of
an intermediate metallicity
cluster like NGC 6752 (
) as well as that of
47 Tuc (
), taking Zinn (1985) metallicity
values.
On the other hand, Liu et al. (1994) derived a metallicity
similar to that of 47 Tuc, and Bica et al. (1997) obtained
from integrated spectra.
Gathering the available results we adopt 
.
Given the uncertainty in metallicity, in order to derive
reddening, we
compared the V, I CMD of
Terzan 10 both with 47 Tuc and NGC 6752 ones (Bica et al. 1994;
Rosino et al. 1997).
Comparing to 47 Tuc we obtain E(V-I) = 3.30,
which converts to E(B-V) = 2.48
(E(V-I)/E(B-V) = 1.33, Dean et al. 1978), whereas
E(V-I) = 3.07 and E(B-V) = 2.31 using NGC 6752.
A mean value of 
is adopted.
The ratio
of selective-to-total extinction R = AV/E(B-V) is dependent
on metallicity and reddening.
A dependence of R on the metallicity for giants was shown by
Grebel & Roberts (1995). For the globular clusters we assumed
,
valid for 
, and R = 3.1 for 
.
Using also the reddening
dependence 
(Olson 1975), we obtain
R = 3.2 and 
.
Using the BG magnitude and colour, and the mean loci
of the reference clusters (47 Tuc, NGC 6752),
we conclude that the HB is at the detection
threshold of our data. The HB level should be
,
while the colour of the RGB at the HB level
should be
.
For the distance derivation we adopt
a relation for the absolute magnitude
of the HB as a function of metallicity by
Jones et al. (1992), with a zero point shift
as explained in Guarnieri et al. (1997):
, resulting MV = 0.82.
The derived absolute distance modulus is
(m-M)0 = 13.40 and the distance 
kpc.
The Galactocentric coordinates are X = 3.2 (X>0
refers to our side of the Galaxy),
Y = 0.4 and Z = -0.2 kpc, having adopted for the distance of
the Galactic center 
kpc (Reid 1993).