Observed rotation period vs. the expected rotation period from the value of log R_HK (upper panel) and R_X (lower panel), as derived by using the Mamajek & Hillenbrand (2008) calibrations. Filled circles represent stars with confirmed or lilely rotation period. Empty circles represent stars with uncertain rotation period. The outlier in the upper panel is represented by HIP 79958. As the measured rotation period appears to be robust, the discrepancy may be due to poor sampling of H and K variability or due to calibration issues resulting from the low temperature of the star. See Appendix B for further details.