We present a BV I optical photometric study of the old open cluster Ruprecht 6 using the data obtained with the SMARTS 1.0 m telescope at the CTIO, Chile. Its color-magnitude diagrams show the clear existence of the main-sequence stars, whose turn-o point is located around V  18:45 mag and B - V  0:85 mag. Three red clump (RC) stars are identied at V = 16:00 mag, I = 14:41 mag and B - V = 1:35 mag. From the mean Ks-band magnitude of RC stars (Ks = 12:39  0:21 mag) in Ruprecht 6 from 2MASS photometry and the known absolute magnitudes of the RC stars (MKS = - :595  0:025 mag), we obtain the distance modulus to Ruprecht 6 of (m -M)0 = 13:84  0:21 mag (d = 5:86  0:60 kpc). From the (J - Ks) and (B - V ) colors of the RC stars, comparison of the (B - V ) and (V - I) colors of the bright stars in Ruprecht 6 with those of the intrinsic colors of dwarf and giant stars, and the PARSEC isochrone ttings, we derive the reddening values of E(B - V ) = 0:42 mag and E(V - I) = 0:60 mag. Using the PARSEC isochrone ttings onto the color-magnitude diagrams, we estimate the age and metallicity to be: log(t) = 9:50  0:10 (t = 3:16  0:82 Gyr) and [Fe/H] = -0:42  0:04 dex. We present the Galactocentric radial metallicity gradient analysis for old (age > 1 Gyr) open clusters of the Dias et al. catalog, which likely follow a single relation of [Fe/H] = (-0:034  0:007)RGC + (0:190  0:080) (rms = 0.201) for the whole radial range or a dual relation of [Fe/H] = (-0:077  0:017)RGC + (0:609  0:161) (rms = 0.152) and constant ([Fe/H]  -0:3 dex) value, inside and outside of RGC  12 kpc, respectively. The metallicity and Galactocentric radius (13:28  0:54 kpc) of Ruprecht 6 obtained in this study seem to be consistent with both of the relations.

Abstract
1. INTRODUCTION
2. OBSERVATIONS AND DATA REDUCTION
3. RESULTS
    3.1. Cluster Center and Size
    3.2. Color-Magnitude Diagrams
    3.3. Distance Estimation
    3.4. Reddening Estimation
    3.5. PARSEC Isochrone Fittings
4. DISCUSSION
    4.1. Red Clump Stars
    4.2. Galactocentric Metallicity Distribution
5. SUMMARY
REFERENCES

• Sang Chul Kim(Korea Astronomy and Space Science Institute/Korea University of Science and Technology (UST)/Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory) Corresponding author
• Jaemann Kyeong(Korea Astronomy and Space Science Institute)
• Hong Soo Park(Korea Astronomy and Space Science Institute/Korea University of Science and Technology (UST)/Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory)
• Ilseung Han(Korea Astronomy and Space Science Institute/Kyungpook National University)
• Joon Hyeop Lee(Korea Astronomy and Space Science Institute/Korea University of Science and Technology (UST))
• Dae-Sik Moon(Department of Astronomy and Astrophysics, University of Toronto)
• Youngdae Lee(Korea Astronomy and Space Science Institute)
• Seongjae Kim(Korea Astronomy and Space Science Institute/Kyungpook National University)