  We report on results of spectropolarimetry of the afterglow of the long
gamma-ray burst GRB 191221B, obtained with SALT/RSS and VLT/FORS2, as well as
photometry from two telescopes in the MASTER Global Robotic Network, at the
MASTER-SAAO (South Africa) and MASTER-OAFA (Argentina) stations. Prompt optical
emission was detected by MASTER-SAAO 38 s after the alert, which dimmed from a
magnitude (white-light) of ~10 to 16.2 mag over a period of ~10 ks, followed by
a plateau phase lasting ~10 ks and then a decline to ~18 mag after 80 ks. The
light curve shows complex structure, with four or five distinct breaks in the
power-law decline rate. SALT/RSS linear spectropolarimetry of the afterglow
began ~2.9 h after the burst, during the early part of the plateau phase of the
light curve. Absorption lines seen at ~6010 \r{A} and 5490 \r{A} are identified
with the Mg II 2799 \r{A} line from the host galaxy at z=1.15 and an
intervening system located at z=0.96. The mean linear polarisation measured
over 3400-8000 \r{A} was ~1.5% and the mean equatorial position angle theta ~65
degrees. VLT/FORS2 spectropolarimetry was obtained ~10 h post-burst, during a
period of slow decline (alpha = -0.44), and the polarisation was measured to be
p = 1.2% and theta = 60 degrees. Two observations with the MeerKAT radio
telescope, taken 30 and 444 days after the GRB trigger, detected radio emission
from the host galaxy only. We interpret the light curve and polarisation of
this long GRB in terms of a slow-cooling forward-shock.
