Example of fit of the nitrogen lines in HD 57682. The observed spectrum is the black solid line. The smooth colored lines are models with the best fit parameters and different values of N/H. The χ² function is evaluated using these models to constrain the value of surface nitrogen content. In the present case, N/H = 1.3    ×    10^-4 is the preferred value (see Fig. 2, red curve). More models with additional N/H values have been calculated but only four are shown for clarity.