NO formation in laminar partially premixed ethane-air flames is investigated as a function of the amount of air introduced into the central fuel tube of an annular co-flow burner. The NO emission index at the exhaust is determined by chemiluminescent detection while the local NO number density is measured by laser-saturated fluorescence. The measurements are taken in flames with an overall equivalence ratio of 0.5, fuel-tube equivalence ratios varying from 1.1 - 10, and a fixed fuel flowrate of 0.15 slpm. Local NO number densities are measured as a function of both radial position and height above the burner. An intermediate dual-flame pattern is identified which minimizes the NO emission index. NO production is found to occur primarily between an inner premixed and an outer nonpremixed flame front, which constitutes the dual-flame structure. These results suggest that the optimum burner-tube equivalence ratio occurs because of a compromise between prompt and thermal formation of NO in the predominantly premixed and nonpremixed fame regions, respectively.