  Magnetic wind confinement has been proposed as one explanation for the
complex wind structures of supergiant stars of spectral types B and A.
Observational investigation of this hypothesis was undertaken using
high-resolution ({\lambda}/{\Delta}{\lambda} {\sim} 65,000) circular
polarization (Stokes V ) spectra of six late B and early A type supergiants
({\beta} Ori, B8Iae; 4 Lac, B9Iab; {\eta} Leo, A0Ib; HR1040, A0Ib; {\alpha}
Cyg, A2Iae; {\nu} Cep, A2Iab), obtained with the instruments ESPaDOnS and
Narval at the Canada-France-Hawaii Telescope and the Bernard Lyot Telescope.
Least Squares Deconvolution (LSD) analysis of the Stokes V spectra of all stars
yields no evidence of a magnetic field, with best longitudinal field 1{\sigma}
error bars ranging from {\sim}0.5 to {\sim}4.5 G for most stars. Spectrum
synthesis analysis of the LSD profiles using Bayesian inference yields an upper
limit with 95.4% credibility on the polar strength of the (undetected) surface
dipole fields of individual stars ranging from 3 to 30 G. These results
strongly suggest that magnetic wind confinement due to organized dipolar
magnetic fields is not the origin of the wind variability of BA supergiant
stars. Upper limits for magnetic spots may also be inconsistent with magnetic
wind confinement in the limit of large spot size and filling factor, depending
on the adopted wind parameters. Therefore, if magnetic spots are responsible
for the wind variability of BA supergiant stars, they likely occupy a small
fraction of the photosphere.
