Light-element abundances in the weak G-band star HR 6766

A high-resolution spectroscopic study of the weak G-band star HR 6766 has been carried out. Atomic line equivalent widths and broad-band photometric colors have been used to derive T/sub eff/=4750 +- 100 K, log g=2.0 +- 0.4, and v/sub t/=2.0 +- 1.0 km s-1. All iron-peak elements have approximately the same abundance, [M/H]=-0.15 +- 0.25. A spectrum synthesis analysis of the CH G band yields [C/H]=-1.49, an underabundance by a factor of 10 relative to a typical giant of similar luminosity. From individual lines of the CN red system and the CN violet band heads, an abundance [N/H]=+0.23 is obtained. Oxygen is nearly normal; the forbidden lines are used to derive [O/H]=-0.38. The CN red system lines have been used to obtain 12C/13C=4.1 +- 1.5 and 14N/15/N>10. Finally, log Li=+0.75 is derived, and no solid evidence for the presence of 6Li is found.HR 6766 is compared with other weak G-band stars, and it is found that all may have the common characteristics of low 12C13C ratios and strong lithium features. The abundances of the CNO group of elements in HR 6766 are consistent with a heavy expure to the CNO cycle in its interior, followed by mixing of the burning products to its surface. Various scenarios for accomplishing this in a low-luminosity giant star (log L/L/sub sun/=1.9) are discussed