THE ASTRONOMICAL JOURNAL    VOLUME 112, NUMBER 3, PAGE 1054      SEPTEMBER 1996

               THE HORIZONTAL BRANCHES OF GLOBULAR CLUSTERS. I.
                    THE COLOR-MAGNITUDE DIAGRAM OF NGC 6426

                         ROBERT ZINN AND SYDNEY BARNES

             Department of Astronomy, Yale University, Box 208101,
                         New Haven, Connecticut 06511
          Electronic mail: zinn@astro.yale.edu, barnes@astro.yale.edu


                                   ABSTRACT

    A color-magnitude diagram of the globular cluster NGC 6426 has been
    constructed from photometry in the V and I passbands.  The cluster has
    the steep giant branch indicative of a very metal-poor composition.
    Following the technique of Sarajedini [AJ, 107, 618 (1994)], we derive
    [Fe/H] = -2.31 +/- 0.10 and E(V-I) = 0.54 +/- 0.03 [E(B-V) = 0.40 +/-
    0.02] from a comparison with the giant branches of other globular
    clusters.  These values are very close to the ones obtained from
    photometry of the cluster's integrated light, and the weighted means of
    our results and the previous ones are [Fe/H] = -2.28 +/- 0.09 and
    E(B-V) = 0.41 +/- 0.02.  The mean V magnitude of the horizontal branch
    (HB) at the instability strip is 18.12 +/- 0.05, from which we estimate
    that NGC 6426 lies 17.4 kpc from the Sun, 11.5 kpc from the galactic
    center, and 4.9 kpc from the galactic plane.  NGC 6426 resembles NGC
    5466 and M68 in both [Fe/H] and HB morphology.  The HB of NGC 6426
    overlaps the one in M15 from the red HB to M15's gap in the blue HB,
    but there is no evidence of significant numbers of faint blue HB stars
    in NGC 6426 that are analogous to the ones constituting the "extended
    HB" in M15.