Until 1980, only a handful of low-mass, active pre-main sequence (pms) stars had known rotation velocities (vsini) /1/. Since then, increasingly sensitive detectors coupled to large telescopes led to high–resolution (a few 104) spectroscopic studies of these faint stars (mv= 10–13), with S/N ratio of the order of 100. The measurement ofvsinifor large samples of pms stars that resulted brought new insights on various pressing questions related to stellar formation and early stellar evolution :

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... w do the rotation rates of pms stars compare with those expected from models of stellar formation ? how does the stellar angular momentum change during pms evolution ? is pms activity linked with rotation as would be expected if activity were triggered by magnetic processes ?