Abundance of phosphorus, A(P), vs. the abundance of magnesium, A(Mg), for stars in our sample. Excluding the two s-rich stars from the sample, we estimate a slope of 0.8 ± 0.1 using linear regression, with the 95% confidence interval marked in blue. This points to a primary behavior of phosphorus similar to magnesium. We adopted solar abundance values of 5.36 and 7.53 for phosphorus and magnesium, respectively, from Grevesse et al. (2007).