Simulations (thin lines) of the X-ray luminosity vs. rotation period (left) and of the X-ray to bolometric luminosity ratio vs. Rossby number (right) for stars with masses between 0.4 and 1.2 M_⊙ having an initial period of rotation of 1.1 days on the ZAMS. Red squares, blue circles, and grey triangles represent measurements of M 34 stars (see Gondoin 2012) located respectively on the C sequence, on the I sequence, and in the gap between these two sequences.