HR 7355 - another rapidly braking He-strong CP star?
  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 HR
7355 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 HR 7355 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 HR 7355 is
the most rapidly rotating CP star known. Our period analyses reveal a possible
lengthening of the rotational period with dP/dt/P = 2.4(8)x 10^-6/yr. We
conclude that the shape and amplitude of HR 7355 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.
