2010A&A...511L...7M

Strong meridional mixing induced by rapid rotation is one reason why all hot main-sequence stars are not chemically peculiar. However, the finding that the He-strong CP star HR 7355 is a rapid rotator complicates this concept. Our goal is to explain the observed behaviour of HR 7355 based on period analysis of all available photometry.Over two years, we acquired 114 new BV observations of HR7355 at observatories in Arizona, U.S.A and Cape Town, South Africa. We performed period analyses of the new observations along with new analyses of 732 archival measurements from the Hipparcos and ASAS projects. We find that the light curves of HR7355 in various filters are quite similar, with amplitudes 0.035(4), 0.036(4), and 0.038(3)mag in B, Hp, and V, respectively. The light curves are double-peaked, with unevenly deep minima. We substantially refine the rotational period to be P=0.5214410(4)d, indicating that HR7355 is the most rapidly rotating CP star known. Our period analyses reveal a possible lengthening of the rotational period with {dot}(P)/P=2.4(8)x10^–6/yr. We conclude that the shape and amplitude of HR7355 light curves are typical of magnetic He-strong CP stars, for which light variations are the result of photospheric spots on the surface of a rotating star. We hypothesise that the light variations are caused mainly by an uneven distribution of overabundant helium on the star's surface. We briefly describe and discuss the cause of the rapid rotational braking of the star.