  Magnetic activity changes the gravito-acoustic modes of solar-like stars and
in particular their frequencies. There is an angular-degree dependence that is
believed to be caused by the non-spherical nature of the magnetic activity in
the stellar convective envelope. These changes in the mode frequencies could
modify the small separation of low-degree modes (i.e. frequency difference
between consecutive quadrupole and radial modes), which is sensitive to the
core structure and hence to the evolutionary stage of the star. Determining
global stellar parameters such as the age using mode frequencies at a given
moment of the magnetic activity cycle could lead to biased results. Our
estimations show that in general these errors are lower than other systematic
uncertainties, but in some circumstances they can be as high as 10% in age and
of a few percent in mass and radius. In addition, the frequency shifts caused
by the magnetic activity are also frequency dependent. In the solar case this
is a smooth function that will mostly be masked by the filtering of the
so-called surface effects. However the observations of other stars suggest that
there is an oscillatory component with a period close to the one corresponding
to the acoustic depth of the He II zone. This could give rise to a
misdetermination of some global stellar parameters, such as the helium
abundance. Our computations show that the uncertainties introduced by this
effect are lower than the 3% level.
