We have used ESO telescopes at La Silla and the Hubble Space Telescope ( HST ) in order to obtain accurate B , V , I CCD photometry for the stars located within 200 \arcsec ( \simeq 2 half-mass radii , r { { } _ { h } } = 1.71 \arcmin ) from the center of the cluster NGC 6101 .
Color-Magnitude Diagrams ( CMDs ) extending from the red-giant tip to about 5 magnitudes below the main-sequence turnoff MSTO ( V = 20.05 \pm 0.05 ) have been constructed .
The following results have been obtained from the analysis of the CMDs : a ) The overall morphology of the main branches confirms previous results from the literature , in particular the existence of a sizeable population of 73 “ blue stragglers ” ( BSS ) , which had been already partly detected ( 27 ) .
They are considerably more concentrated than either the subgiant branch ( SGB ) or the main sequence ( MS ) stars , and have the same spatial distribution as the horizontal branch ( HB ) stars ( 84 \% probability from K-S test ) .
An hypothesis on the possible BSS progeny is also presented .
b ) The HB is narrow and the bulk of stars is blue , as expected for a typical metal-poor globular cluster .
c ) The derived magnitudes for the HB and the MSTO , V _ { ZAHB } = 16.59 \pm 0.10 , V _ { TO } = 20.05 \pm 0.05 , coupled with the values E ( B-V ) = 0.1 , [ Fe/H ] = -1.80 , Y = 0.23 yield a distance modulus ( m - M ) _ { V } = 16.23 and an age similar to other “ old ” metal-poor globular clusters .
In particular , from the comparison with theoretical isochrones , we derive for this cluster an age of 13 Gyrs .
d ) By using the large statistical sample of Red Giant Branch ( RGB ) stars , we detected with high accuracy the position of the bump in the RGB luminosity function .
This observational feature has been compared with theoretical prescriptions , yielding a good agreement within the current theoretical and observational uncertainties .