  About 10% of upper main sequence stars are characterised by the presence of
chemical peculiarities, often found together with a structured magnetic field.
The atmospheres of most of those chemically peculiar stars present surface
spots, leading to photometric variability caused by rotational modulation. The
study of the light curves of those stars therefore, permits a precise
measurement of their rotational period, which is important to study stellar
evolution and to plan further detailed observations. We analysed the light
curves of 1028 chemically peculiar stars obtained with the STEREO spacecraft.
We present here the results obtained for the 337 magnetic chemically peculiar
stars in our sample. Thanks to the cadence and stability of the photometry,
STEREO data are perfectly suitable to study variability signals with a
periodicity typical of magnetic chemically peculiar stars. Using a matched
filter algorithm and then two different period searching algorithms, we
compiled a list of 82 magnetic chemically peculiar stars for which we measured
a reliable rotational period; for 48 of them this is the first measurement of
their rotational period. The remaining 255 stars are likely to be constant,
although we cannot exclude the presence of long period variability. In some
cases, the presence of blending or systematic effects prevented us from
detecting any reliable variability and in those cases we classified the star as
constant. For each star we classified as variable, we determined temperature,
luminosity, mass and fractional age, but the limited statistics, biased towards
the shorter periods, prevented us from finding any evolutionary trend of the
rotational period. For a few stars, the comparison between their projected
rotational velocity and equatorial velocity let us believe that their real
rotational period might be longer than that found here and previously obtained.
