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The visual binary system 16~Cyg~A+B consists of similar solar twins, but aplanetary companion is detected only in B. An intensive spectroscopicdifferential analysis is carried out to the Sun, 16~Cyg~A, and 16~Cyg~B, withparticular attentions being paid to (i) precisely establishing the differentialatmospheric parameters/metallicity between A and B, and (ii) determining theimportant CNO abundances based on the lines of CH, NH, and OH molecules. Thefollowing results are obtained. (1) The Fe abundances (relative to the Sun) are[Fe/H]^A=+0.09 and [Fe/H]^B=+0.06 (i.e., A is slightly metal-rich than B by+0.03~dex). This lends support to the consequences of recently publishedpapers, while the conclusion once derived by the author (almost the samemetallicity for A and B) is acknowledged to be incorrect. (2) The differentialabundances (Delta[X/H]) of volatile CNO with low T_c (condensation temperature)are apparently lower than those of refractory Fe group elements of higher T_c,leading to a positive gradient in the Delta[X/H] vs. T_c relation being moreconspicuous for A than B. This is qualitatively consistent with previousstudies, though the derived slope is quantitatively somewhat steeper than thatreported by other authors.

On the chemical abundance differences between the solar twin visual binary system 16 Cygni A and B