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\label{apsidal2}Astronomy&AstrophysicsReview (a) Measured internal structure constant as a function of the weighted mean logarithmic mass in each system. The ZAMS relation from the \cite{1995A+AS..109..441C} models for solar metallicity is shown. (b) Deviations of the measured internal structure constant from the ZAMS relation in (a) as a function of the weighted mean surface gravity in each system. The dashed line is for reference; no physical relation is suggested.