Left: X-ray luminosities of the M 34 sample stars vs. rotation periods. Right: X-ray to bolometric luminosity ratios vs. Rossby numbers. The mass of each star is indicated by the size of the associated circle. The dashed lines represent the lower and upper limits of the correlation results L_X vs. P_rot and L_X/L_bol vs. R_o established by Pizzolato et al. (2003) for masses in the range 0.63–0.78 M_⊙ (lower curve) and 1.03–1.10 M_⊙ (upper curve), respectively.