Element stratification in main sequence stars and its effect on stellar oscillations

The element settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars, which cannot be neglected in the computations of stellar structure. These processes where first introduced to account for abundance anomalies in "peculiar stars", but their importance in the so-called "normal" stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, like rotation-induced mixing or mass loss, which increase the settling time scales. We have recently obtained clear evidences that asteroseismology of main sequence solar type stars can give signatures of the chemical variations inside the stars and help for a better understanding of these processes.