tokens: We thank the authors of the atlas9 , marcs , phoenix and drift-phoenix model atmosphere grids to allow us to use their model results for this comparison study, and for their helpful suggestions. We thank Dr Tim-Oliver Husser (Institut für Astrophysik, Georg-August-Universitat Göttingen) for his support with the phoenix models. We also thank Sören Witte and our referee for constructive feedback. ChH highlights financial support of the European Community under the FP7 by an ERC starting grant. IB thanks the Physics Trust of the University of St Andrews for supporting her summer placement. Most literature search was performed using the ADS. Our local computer support is highly acknowledged.
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