  We study electromagnetically induced transparency (EIT) in a heated potassium
vapor cell, using a simple optical setup with a single free-running diode laser
and an acousto-optic modulator. Despite the fact that the Doppler width is
comparable to the ground state hyperfine splitting, transparency windows with
deeply sub-natural line widths and large group indices are obtained. A
longitudinal magnetic field is used to split the EIT feature and induce
magnetooptical anisotropy. Using the beat note between co-propagating coupling
and probe beams, we perform a heterodyne measurement of the circular dichroism
(and therefore birefringence) of the EIT medium. The observed spectra reveal
that lin-par-lin polarizations lead to greater anisotropy than lin-perp-lin. A
simplified analytical model encompassing sixteen Zeeman states and eighteen
\Lamda\ subsytems reproduces the experimental observations.
