Linear polarization observations of P/Halley.

Linear polarization observations of P/Halley (1982i) obtained at the Mont Megantic Observatory during the period October 1985-January 1986 are reported. The range in phase angles covered is 18-55 deg. The polarization vector was perpendicular or parallel to the scattering plane on all occasions, except once when the phase angle was nearly equal to the cross-over value. The polarization as a function of phase angle is qualitatively the same as in other comets and in asteroids, the slope being 0.33 percent/deg for beta smaller than 40 deg. Single scattering by spherical grains does not appear to be able to explain the observations. If one assumes that the dust in comets is similar to dust on the surface of asteroids, then a low albedo, of less than 0.1, is implied.