6 The color-magnitude diagrams

In this section the V vs. (V-I) CMDs for the 39 GGCs and the covered fields are presented.

The same color and magnitude scale has been used in plotting the CMDs, so that differential measures can be done directly using the plots. Two dot sizes have been used, with the bigger ones corresponding to the better measured stars, normally selected on the basis of error ($\leq0.1$) and sharpness parameter (Stetson 1987). In some exceptional cases, a selection on radius is also done in order to make evident the cluster stars over the field stars, or to show differential reddening effects. The smaller size dots show all the measured stars with errors (as calculated by DAOPHOT) smaller than 0.15 mag.

The images of the fields are oriented with the North at the top and East on the left side. As explained in Sect. 2, each field covers $3.77\times3.77 (\hbox{$^\prime$ })^2$, and the overlaps between fields of the same object are about $20-25\%$ of the area. For some clusters, only short exposures were obtained for the central fields.

In the next subsections, we present the single CMDs and clusters, and give some references to the best existing CMDs. This is by no means a complete bibliographical catalog: a large number of CMDs are available in the literature for many of the clusters of this survey, but we will concentrate just on the best CCD photometric works. The tables with the position and photometry of the measured stars will be available via a web interface at IAC and Padova in the near future.

NGC 104 (47 Tucanae).

(Fig. 4)

The cluster 47 Tucanae is (after $\omega$ Centauri) the second brightest globular cluster in the sky, and consequently a lot of work has been done on this object. 47 Tucanae has been often indicated as the prototype of the metal-rich GGCs, characterized by a well populated red HB (RHB) clump and an extended RGB that also in our CMD spans $\sim$2 mag in color from the RHB to the reddest stars at the tip.

A classical CMD of 47 Tucanae is that presented by Hesser et al. ([1987]) where a composite CMD was obtained from the superposition of B and V CCD photometry for the main sequence (MS) and photographic data for the evolved part of the diagram. The same year, Alcaino & Liller ([1987a]) published a BVI CCD photometry. One year later, Armandroff ([1988]) presented the RGB V and I bands photometry for this cluster (together with other five). In 1994, Sarajedini & Norris ([1994]) presented a study of the RGB and HB stars in the B and I bands. Sosin et al. ([1997a]) and Rich et al. ([1997]) have published a B,V photometry based on HST data.

A recent work in the V and I bands has been presented by Kaluzny et al. ([1998]), who focussed their study on the variable stars. They do not find any RR-Lyrae, but many other variables (mostly located in the BSS region), identified as binary stars. As already stated by these authors, a small difference is found between their and our photometry. Indeed, their magnitudes coincide with ours at $\sim 12.5$ mag in both bands, but there is a small deviation from linearity of ~ 0.015 magnitudes per magnitude (with the Kaluzny et al. stars brighter than ours), in both bands (computed from 90 common stars with small photometric errors) within a magnitude range of ~ 3 mag. We are confident that our calibration, within the quoted errors, is correct, as further confirmed by the comparison with other authors for other objects, as discussed below. Although small, these differences could be important in relative measures, if they appear randomly in different CMDs. For example, in this case, the $\Delta V_{\rm TO}^{\rm HB}$ parameter is $\sim0.05$ mag smaller in Kaluzny et al. ([1998]) CMD than in our one, implying, for the 47 Tuc metallicity, an age difference of $\sim0.8$ Gyrs. We want to stress the importance of a homogeneous database for a reliable measurement of differential parameters on the CMDs.

NGC 288 and NGC 362.

(Figs. 5 and 6)

The diagram of NGC 288 is well defined and presents an extended blue horizontal branch (EBHB) which extends from the blue side of the RR-Lyrae region, to just above the TO. Conversely, NGC 362 has a populated RHB with just a few blue HB stars.

These two clusters define one of the most studied second parameter couple: despite their similar metallicities, their HB morphologies are different. Much work have been done on both clusters in order to try to understand the origin of such differences: Bolte ([1989]) and Sarajedini & Demarque ([1990]) in the Band V bands, and Green & Norris ([1990]) in the Band R bands, based on homogeneous CCD photometry, obtain an age difference of $\sim 3$ Gyrs, NGC 288 being older than NGC 362. A similar conclusion is obtained in our study (Paper III), where NGC 362 is found $\sim20\%$ younger than NGC 288. It has also been proposed (e.g. Green & Norris [1990]) that these age differences might be responsible of the HB differences between the two clusters. On the other side, Buonanno et al. ([1998]) and Salaris & Weiss ([1998]) do not find significant age differences. Another B,V photometry of NGC 362 based on HST data is in Sosin et al. ([1997a]).

It might be worth to remark here that, as it will be discussed in Paper II, there are clusters with different HB morphologies, though with the same metallicities and ages (within errors). This means that the analysis of a single couple of GGCs can not be considered conclusive for understanding the second parameter problem, while a large scale study (as that feasible with this catalog) can be of more help.

NGC 1261.

(Fig. 7)

This cluster is the object with the largest distance in our southern hemisphere sample. It is located at $\sim 16$ kpc from the Sun.

Three major CCD CMDs have been published for NGC 1261: Bolte & Marleau ([1989]) in B,V, Alcaino et al. ([1992]) in B,V,R,I, and Ferraro et al. ([1993b]) in the B and V bands.

The CMD is characterized by an HB which is similar to the HB of NGC 1851. From here on, clusters with an HB well populated both on the red and blue side of the RR-Lyrae gap will be named bimodal HB clusters, though a more objective classification would require taking into account the color distribution of stars along the HB including the RR Lyrae (Catelan et al. [1998]). NGC 1261 has a metallicity very close to that of the previous couple; Chaboyer et al. ([1996]), Richer et al. ([1996]) and Rosenberg et al. (Paper III) find that it is younger (similar in age to NGC 362) than the bulk of GGCs. A blue straggler (BS) is clearly visible in Fig. 7.

NGC 1851.

(Fig. 8)

This cluster has a bimodal HB, with very well defined RHB and blue HB (BHB). Also in this case, a BS sequence is visible in Fig. 8. It is curious that, again, a bimodal cluster results to be younger than the GGCs bulk. From the 34 clusters in the present catalog, only 4 result to be surely younger, i.e. the already described NGC 362 and NGC 1261, this cluster, and NGC 2808: three of them have a bimodal HB (cf. Rosenberg et al. [1999] for a detailed discussion). There exist other two recent (V,I) CCD photometries of NGC 1851 by Walker ([1998]) and Saviane et al. ([1998]). The three photometries are all in agreement within the errors, confirming our calibration to the standard system. A CMD of NGC 1851 in the B,V bands from HST is in Sosin et al. ([1997a]). Older CCD photometries are found in Alcaino et al. ([1990a]) (B,V,I bands) and Walker ([1992]) (B,V bands).

NGC 1904 (M 79).

(Fig. 9)

M 79 is the farthest cluster ( $R_{\rm GC}=18.5$ kpc) from the Galactic center in our sample. The main feature in the CMD of Fig. 9 is the EBHB. Previous CMDs from CCD photometry are in Heasley et al. ([1983]) (U,B,V bands), Gratton & Ortolani ([1986]) (B,V bands), Ferraro et al. ([1993a]) (B,V bands), Alcaino et al. ([1994]) (B,V,R,I bands), and Kravtsov et al. ([1997]) (U,B,V bands), and the B,V photometry from HST in Sosin et al. ([1997a]).

NGC 2298.

(Fig. 10)

This cluster is poorly sampled, particularly for the bright part of the diagram (due to problems with a short exposure). Only four BHB stars are present in the HB region. Recent photometric works on this object are in Gratton & Ortolani ([1986]) (B,Vbands), Alcaino & Liller ([1986a]) (B,V,R,I bands), Janes & Heasley ([1988]) (U,B,V bands), and Alcaino et al. ([1990b]) (B,V,R,I bands).

NGC 2808.

(Fig. 11)

This cluster has some differential reddening (Walker 1999), as it can be inferred also from the broadening of the sequences in the CMD of Fig. 11, and a moderate field contamination. The most interesting features of the CMD are the bimodal HB and the EBHB tail with other two gaps, as extensively discussed in Sosin et al. ([1997b]). As previously discussed, NGC 2808 is another bimodal HB cluster at intermediate metallicity with a younger age (Rosenberg et al. [1999]). Apart from the already quoted B, and V band photometry from HST data by Sosin et al. ([1997b]), there are many other CCD photometries: Gratton & Ortolani ([1986]) (B,V bands), Buonanno et al. ([1989]) (B,V bands), Ferraro et al. ([1990]) (B,V bands), Alcaino et al. ([1990c]) (B,V,R,I bands), Byun & Lee ([1993]), Ferraro et al. ([1997]) (V,I bands), and more recently Walker ([1999]) (B,V bands).

E3.

(Fig. 12)

This cluster is one of the less populated clusters in our Galaxy, resembling some Palomar-like globular as Pal 1 (Rosenberg et al. [1998]). As in Pal 1, there are no HB stars in the CMD, and the entire population of observed stars is smaller than 1000 objects. E3 is suspected to have a metallicity close to that of Pal 1. From the $\delta (V-I)_{\rm @2.5}$)_@2.5 (Paper III) measured in Fig. 12, E3 is coeval with the other GGCs of similar metallicity, though the result is necessarily very uncertain, due to the high contamination and the small number of RGB stars. E3 is the cluster with the better defined MS binary sequence (Veronesi et al. [1996]), which can be also seen in Fig. 12. Previous CCD CMDs are in McClure et al. ([1985]) (B,V bands), Gratton & Ortolani ([1987]) (B,Vbands), and Veronesi et al. ([1996]) (B,V,R,I bands).

NGC 3201.

(Fig. 13)

The two lateral fields presented in Fig. 13 were observed in both runs, in order to test the homogeneity of the data and instrumentation (see Sect. 4). The HB of NGC 3201 has a bimodal appearance, though it is not younger than the bulk of GGCs of the same metallicity group, at variance with the previously discussed cases. It has a small differential reddening. A blue straggler (BS) sequence is visible in Fig. 13. Previous CCD studies of this cluster include Penny ([1984]) (B,V,Ibands), Alcaino et al. ([1989]) (B,V,R,I bands), Brewer et al. ([1993]) (U,B,V,I bands) and Covino & Ortolani ([1997]) (B,V bands).

NGC 4372.

(Fig. 14)

The principal characteristic of the CMD of this cluster is the broadening of all the sequences, consequence of the high differential reddening, probably due to the Coal-sack Nebulae. In the CMD of Fig. 14 the darker dots are from the stars in the lowest reddening region (south east) of the observed fields. We have computed the reddening field for this cluster from the shift of the CMDs obtained in different positions, finding that it is homogeneously distributed in space and quite easy to correct by a second order polynomial surface. Two previous CCD photometries can be found in Alcaino et al. ([1991]) (B,V,R,I bands) and Brocato et al. ([1996]) (B,V bands).

NGC 4590 (M 68).

(Fig. 15)

This cluster is probably the lowest metallicity cluster of the present sample. It has a well defined CMD, with an HB populated on both sides of the instability strip, and including some RR-Lyrae stars. It has sometimes been classified as one of the oldest GGCs (Salaris et al. [1997]), and, in fact, we find that M 68 is old, though coeval with the rest of the metal poor clusters (Paper III). Other CCD CMDs for this cluster are in McClure et al. ([1987]) (B,V bands), Alcaino et al. ([1990d]) (B,V,R,I bands) and Walker ([1994]) (B,V,I bands).

NGC 4833.

(Fig. 16)

NGC 4833 is another metal-poor cluster, with an extended BHB, likely with gaps, for which we have not found any previous CCD photometry.

NGC 5139 ($\omega$ Centauri).

(Fig. 17)

NGC 5139 is the intrinsically brightest cluster in our Galaxy. Apart from this, there are many other properties of $\omega$ Centauri which make it a very particular object. Its stellar population shows metallicity variations as large as $\sim1.5$ dex from star to star (Norris et al. [1996]). Its overall properties suggest that this clusters could have a different origin from the bulk of GGCs. It has an extended BHB and probably numerous BSS. The broad sequences in the CMD are mainly due to the metallicity variations though likely there is some differential reddening in the field of $\omega$Centauri. Due to its peculiarities, $\omega$ Centauri has been (and is!) extensively studied; there is a large number of photometries, and we cannot cite all of them. The most recent and interesting CCD CMDs are in: Alcaino & Liller ([1987b]), who present a multi-band (B,V,R,I) photometry, but poorly sampled, specially for the evolved part of the diagram; Noble et al. ([1991]) present a deep B,V diagram, where the MS is well sampled, but the RGB is not so clear and only 3-5 stars are present in the HB; Elson et al. ([1995]) present a HST V,I photometry of the MS; Lynga ([1996]) presents a BVRI study of the evolved part of the diagram ($\sim 2$ mag below the HB); Kaluzny et al. ([1996], [1997]) present a V, I CMD covering more than 10⁵ stars.

NGC 5897.

(Fig. 18)

NGC 5897 is a metal poor cluster with a blue, not extended HB, typical for its metallicity. All the sequences of Fig. 18 are well defined and populated, including a BS sequence. Two CCD photometric studies exist for this cluster: Sarajedini ([1992]) (B,V bands) and Ferraro et al. ([1992]) (U,B,V,I bands).

NGC 5927.

(Fig. 19)

NGC 5927 has the highest metallicity among the objects of our catalog. It has, as most of the GGCs with [Fe/H]>-0.8, a well populated red horizontal branch (RHB), and an extended RGB, which, in our CMD, covers more than $\sim$2.5 mag in (V-I), from the RHB (partially overlapped with the RGB) to the reddest stars of the RGB tip. It has a high reddening, possibly differential, judging from the broadening of the RGB, and, due to its location (projected towards the Galactic center), the field object contamination (disk and bulge stars) is very high. Previous CCD photometries are in Friel & Geisler ([1991]) (Washington photometry), Sarajedini & Norris ([1994]) (B,V bands), Samus et al. ([1996]) (B,V,I bands), Sosin et al. ([1997a]), and Rich et al. ([1997]) (HST B,V bands).

NGC 5986.

(Fig. 20)

To our knowledge, this is the first CCD photometry for this cluster. NGC 5986 is an intermediate metallicity cluster, but with a metal-poor like HB. The broadening of the CMD suggests some differential reddening. Contamination by field stars is clearly visible, as expected on the basis of the position within the Galaxy of this cluster.

NGC 6093 (M 80).

(Fig. 21)

NGC 6093 is a bright and moderately metal poor cluster, and one of the densest globular clusters in the Galaxy. It has an EBHB, which extends well below the TO as clearly visible also in the CMD of Fig. 21, with gaps (Ferraro et al. 1998). Three recent CCD photometries that cover the entire object, with CMD from the brightest stars to above the TO exist for this cluster: Brocato et al. ([1998]) (B,V bands) and Ferraro et al. ([1998]) (HST U,V, and far-UV (F160BW) bands). A ground-based multicolor U,B,V,I CCD CMD has been published also by Alcaino et al. ([1998]).

NGC 6101.

(Fig. 22)

NGC 6101 was observed under not very good seeing conditions, and this is the reason for the brighter limiting magnitude. Its CMD has the morphology expected for a metal-poor cluster: the HB is predominantly blue, and the giant branch is steep. In Fig. 22 we note that, starting from the BSS sequence, there is a sequence of stars parallel to the RGB on its blue side. In view of the position of the cluster (l,b) = (318, -16) these can unlikely be bulge stars; it is possible that on the same line of sight there is an open cluster, though the slope of the two RGBs are quite similar, implying an unlikely similar metallicity. A larger field coverage of NGC 6101 is desirable. The only previous CCD photometry that exists for this cluster is the B and V study by Sarajedini & Da Costa ([1991]), which shows these stars in the same CMD location. However, being the background-foreground stellar contamination heavier, the sequences we discussed can hardly be seen.

NGC 6121 (M 4).

(Fig. 23)

This cluster is the closest GGC, located approximately at $\sim2.2$ kpc from the Sun, though, due to the large reddening caused by the nebulosity in Scorpio-Ophiuchus, it has an apparent visual distance modulus larger than NGC 6397. The reddening is differential, though (as in the case of NGC 4372) it is homogeneously distributed in space. The mean regions of the CMD can be improved using an appropriate second order polynomial fit to the reddening distribution, at least on the two fields shown in Fig. 23. The stars from the southern field have been plotted as darker dots; they are located on the redder (more reddened) part of the CMD. The two most recent CMD of M 4 are in Ibata et al. ([1999]) (V,I,U filters) and Pulone et al. ([1999]), who present (near IR) HST studies of the faint part of the MS and of the WD sequence. Other recent CMDs from the RGB tip to below the MSTO are in Alcaino et al. ([1997a]), who presented an UBVI CCD photometry, and Kanatas et al. ([1995]), who obtained a composite (B,V) CMD from V ~ 12 to V ~ 25.

NGC 6171 (M 107).

(Fig. 24)

Previous CCD studies of NGC 6171 are the (J,K) and (B,V)photometry by Ferraro et al. ([1995] and [1991], respectively). This cluster is affected by a moderate reddening, which could be slightly differential. It has a RHB, with a few stars bluer than the instability strip blue edge.

NGC 6266 (M 62).

(Fig. 25)

This cluster is located very close to the Galactic center, and it has a high differential reddening. It seems to have both a RHB and a BHB resembling the HB of NGC 1851. Previous B,V bands CCD works are in Caloi et al. ([1987]), and Brocato et al. ([1996]). A de-reddened CMD and RR-Lyrae stars are also studied in Malakhova et al. ([1997]).

NGC 6304.

(Fig. 26)

NGC 6304 is a high metallicity cluster very close to the Galactic center, and has one of the highest reddenings in our sample. It has some disk and bulge star contamination. There is a second RGB fainter and redder than the main RGB (bulge star contamination or a more absorbing patch?), but the most noticeable feature is the extremely long RGB. The reddest star of its RGB is located $\sim$3.7 mag redward from the RHB! To our knowledge, this is the most extended RGB known for a GGC. The most recent CCD CMD for this cluster comes from the Vand K photometry by Davidge et al. ([1992]) which covers the hottest RGB stars and the HB.

NGC 6352.

(Fig. 27)

NGC 6352 is another high metallicity bulge GGC, with a CMD typical of a cluster with this metal content. The most recent CCD study on this cluster is in Fullton et al. ([1995]), where a $VI_{\rm c}$ CMD from HST data combined with ground-based observations is presented. Another study of the RGB and HB regions of this cluster is presented by Sarajedini & Norris ([1994]) in the B,V bands.

NGC 6362.

(Fig. 28)

NGC 6362 presents a well defined CMD with a bimodal HB. The most recent CMD on this cluster is given by Piotto et al. ([1999]), who present observations of the center of the cluster obtained with the HST/WFPC2 camera in the B and Vbands. The only previous ground-based CCD photometry is in Alcaino & Liller ([1986b]). Our field has been also observed in the same filters by Walker (priv. comm.), who made available to us his data for a cross-check of the photometric calibration. We find that the two photometries agree within the errors. In particular, we found a zero point difference of 0.02 mag for the V band and 0.01 mag for the I band, with a negligible -0.001 color term difference between Walker and our data. These discrepancies are well within the uncertainties, and allow to further confirm our calibration to the standard (Landolt [1992]) system.

NGC 6397.

(Fig. 29)

This cluster is the GGC with the smallest apparent distance modulus. Cool et al. ([1996]) and King et al. ([1998]) present an extremely well defined CMD of the main sequence of this cluster, from HST data, from just below the TO down to I=24.5, which correspond to a mass of less than $0.1~M_\odot$. Other HST studies on this cluster have been presented by Burgarella et al. ([1994]), De Marchi & Paresce ([1994]), Cool et al. ([1995]) and King et al. ([1995]). Many ground-based CCD data have also been published: Auriere et al. ([1990]), Anthony-Twarog et al. ([1992]) (Stromgren photometry), Lauzeral et al. ([1992], [1993]), Kaluzny ([1997]) (B,V bands) and Alcaino et al. ([1987]: B,V bands; [1997b]: U,B,V,I bands).

NGC 6496.

(Fig. 30)

NGC 6496 is another metal rich GGC which presents an extended RGB. In this case, the reddest stars are $\sim$ 2 mag redder than the RHB. It has also a remarkably tilted RHB, already noted by Richtler et al. ([1994]), who present a CCD (B,V) photometry of this cluster; Armandroff ([1988]) gives (V,I) CCD photometry. A tilted RHB can be noted not only in this CMD, but also in the CMDs of most of the very metal-rich clusters of our sample. Such a feature is usually not present in the canonical models. The RHB is well populated, and there are two stars located on the BHB region. This is quite unusual considering the metallicity of NGC 6496, and it would be interesting to study the membership and to obtain a CMD on a larger field. Another CCD photometry of this cluster is in Friel & Geisler ([1991]) in the Washington system. Sarajedini & Norris ([1994]) present a Band V photometry for the RGB and HB region.

NGC 6541.

(Fig. 31)

NGC 6541 is located rather close to the Galactic center, and this explains the high field star contamination of the CMD. It has a BHB, as expected from its metal content. The only previous CCD study of this cluster is the multicolor photometry by Alcaino et al. ([1997c]).

NGC 6544.

(Fig. 32)

This is an example of a terrible "spotty'' field with a high (the highest in our sample) and highly differential reddening, due to the location of NGC 6544, which is very close to the Galactic plane and projected towards the Galactic center. Interestingly enough, despite its intermediate metallicity, there are only BHB stars. Probably, the use of the HST in this case is almost inevitable if we want to estimate the age of this kind of clusters. We have not found any previous CCD photometry of this cluster.

NGC 6624.

(Fig. 33)

Another member of the metal-rich group is presented in Fig. 33. Despite of being the cluster closest to the Galactic center, NGC 6624 has a moderate field star contamination, and a very well defined RGB and RHB. The reddest stars of the RGB are in this case $\sim2.2$ mag redder than the RHB.

Richtler et al. ([1994]) present a B and V CCD CMD of this cluster extending well below the TO, while Sarajedini & Norris ([1994]) present a photometric study of the RGB and HB in the same bands. A B,V CMD from HST data is in Sosin & King ([1995]) and Sosin et al. ([1997a]).

NGC 6626 (M 28).

(Fig. 34)

Again a high differential reddening is present in the field of NGC 6626, which is located close to the Galactic center. NGC 6626 seems to have an extended BHB, and maybe a few RHB stars, though the field star contamination makes it rather difficult to see them. Previous CCD photometry is given by Davidge et al. ([1996]), who present a deep near infrared photometry.

NGC 6637 (M 69).

(Fig. 35)

The CMD of NGC 6637 presents the typical distribution in color for the RGB stars discussed for other metal rich clusters, with the reddest stars $\sim2.4$ mag redder than the RHB. Previous B and V CCD photometry is presented by Richtler et al. ([1994]), and the RGB-HB region is also studied by Sarajedini & Norris ([1994]) in the same bands.

NGC 6638.

Fig. 36

Affected by high differential reddening, the CMD this cluster is not very well defined. However, the HB is clearly populated on both sides of the instability strip, and probably there are many RR-Lyrae. We have not found any previous CCD photometries of this cluster.

NGC 6656 (M 22).

(Fig. 37)

A possible internal dispersion in metallicity has been proposed for M 22. It presents an EBHB with some HB stars as faint as the TO, and several possible RR-Lyrae stars. It is close to the Galactic center and to the Galactic plane, with a high reddening.

Piotto & Zoccali ([1999]) published the most recent study of this cluster. From a combination of HST data and ground based CCD photometry, they produced a CMD extending from the tip of the RGB to below $0.2~M_\odot$. Anthony-Twarog et al. ([1995]) present uvbyCa data for over 300 giant and HB stars, while in Davidge & Harris ([1996]) there is a deep near infrared study.

NGC 6681 (M 70).

(Fig. 38)

NGC 6681 has a predominantly blue HB with a few HB stars on the red side of the instability strip. Brocato et al. ([1996]) present the only other available CCD photometry for this cluster in the Band V bands.

NGC 6717 (Palomar 9).

(Fig. 39)

NGC 6717 is a poorly populated cluster (as most of the "Palomar-like'' objects), and the CMD is contaminated by bulge stars. The RGB is difficult to identify, and its HB is blue, resembling that of NGC 288. Notice that there is a very bright field star close to the cluster, located at the north side of it. Brocato et al. ([1996]) present the first CCD photometry for this cluster; their B and V CMD resembles that of Fig. 39. Recently, Ortolani et al. ([1999]) presented a new CMD, in the same bands, but the CMD branches are more poorly defined.

NGC 6723.

(Fig. 40)

NGC 6723 has both a red and blue HB, and the overall morphology is typical of a cluster of intermediate metallicity. Alcaino et al. ([1999]) present the most recent CCD study (multicolor photometry), with a CMD extending down to $V\sim21$. Fullton & Carney ([1996]) have obtained a deep B and V photometry, extending to $V\sim 24$ 24, though the results of this study have not been completely published, yet.

NGC 6752.

(Fig. 41)

NGC 6752 has been largely studied in the past. It has a very well defined EBHB. Penny & Dickens ([1986]) presented a B and V CCD study from a combination of data from two telescopes, and published a CMD from the RGB tip to $V\sim$ 24 mag, though with a small number of measured stars. In the same year, Buonanno et al. ([1986]) present a CMD in the same bands for stars from $\sim1$ mag above the TO to $\sim5$ mag below it. More recently, Renzini et al. ([1996]) and Rubenstein & Baylin ([1997]) published a CMD from HST data.

NGC 6809 (M 55).

(Fig. 42)

Also the CMD of NGC 6809 is typical for its (low) metallicity. A very well defined BS sequence is visible in Fig. 42. The most recent CCD study is in Piotto & Zoccali ([1999]), who study the cluster luminosity function based on deep HST data combined with ground-based CCD data for the evolved part of the CMD. Zaggia et al. ([1997]) present Vand I CCD photometry of $\sim34000$ stars covering an entire quadrant of the cluster (out to ~ 1.5 times the tidal radius) down to $V\sim21$. Mateo et al. ([1996]) and Fahlman et al. ([1996]) presented photometric datasets of M 55 that have been mainly used to study the age and the tidal extension of the Sagittarius dwarf galaxy. Mandushev et al. ([1996]) published the first deep (down to V ~ 24.5) photometry of the cluster.