1. Introduction

2. Observations and stellar parameters

3. Abundance analysis
3.1. Line synthesis
3.2. Abundance uncertainties

4. Results
4.1. Carbon
4.2. Silicon

5. Discussion and conclusions
5.1. Abundance correlations with stellar parameters
5.2. The outliers in our sample
5.3. Star-to-star scatters and mixing of the interstellar medium