Top: Minimum value of ΔE∕E against the squared fraction between the binary period P_bin and the critical period P_crit defined as the period the binary would have if the secondary is rotating at its critical rotation velocity under the assumption of tidal lockment. Bottom: plotted against β defined in Eq. (7); the rightmost circle corresponds to HW Vir. Red triangles are the systems falling below the horizontal dashed line at Fig. 3.