tokens: We are grateful to R. Trampedach for clarifications about his results, and for making available the routine to calculate Rosseland opacities. We also thank J. Christensen-Dalsgaard for interesting discussions on this topic, and the anonymous referee for comments that have improved the presentation of our results. S.C. gratefully acknowledges financial support from PRIN-INAF2014 (PI: S. Cassisi).
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