THE MAGNETIC-FIELD GEOMETRY AND ROTATIONAL PERIOD OF THE BP STAR HD-200311

We present a model of the magnetic field geometry of the Bp star HD 20 0311 based upon new mean longitudinal magnetic field measurements and previously published mean magnetic field modulus measurements by Mathy s ct al. This is the fourth in a series of papers aimed at constructin g magnetic field models for peculiar stars throughout the F0-B2 temper ature range. The 21 longitudinal magnetic field measurements indicate the presence of a strong and variable line-of-sight magnetic field. Th ese data do not define a smooth curve of variation when phased with th e 26.0042-d period found by Adelman (1997) from Stromgren photometry. However, when phased with a period of 52.0084 d (twice Adelman's perio d). the longitudinal field data define a sinusoidal curve with extrema of -1.8 and + 1.8 kG. The mean magnetic field modulus measurements de scribe an approximately sinusoidal curve when phased with this same pe riod. Adelman's photometry, when phased with this period, describes a double-wave variation with approximately equal maxima, one of which oc curs close in phase to the longitudinal field maximum. This is consist ent with an inhomogeneous chemical abundance distribution that is symm etric about tile magnetic field axis, and similar in both magnetic hem ispheres. The phased magnetic field data have been modelled assuming a n oblique decentred magnetic dipole, The best-fitting oblique rotator parameters for this model are rotational asis inclination i = 28 degre es +/- 8 degrees or 90 degrees +/- 8 degrees, magnetic axis obliquity angle beta = 90 degrees +/- 8 degrees or 28 degrees +/- 8 degrees, pol ar strength of the magnetic dipole (when centred)B-d = -12.8 +/- 1 kG, and dipole decentring parameter a = 0.09 +/- 0.04.