  We present high speed photometry and high resolution spectroscopy of the
eclipsing post common envelope binary QS Virginis (QS Vir). Our UVES spectra
span multiple orbits over more than a year and reveal the presence of several
large prominences passing in front of both the M star and its white dwarf
companion, allowing us to triangulate their positions. Despite showing small
variations on a timescale of days, they persist for more than a year and may
last decades. One large prominence extends almost three stellar radii from the
M star. Roche tomography reveals that the M star is heavily spotted and that
these spots are long-lived and in relatively fixed locations, preferentially
found on the hemisphere facing the white dwarf. We also determine precise
binary and physical parameters for the system. We find that the 14,220 +/- 350K
white dwarf is relatively massive, 0.782 +/- 0.013Ms, and has a radius of
0.01068 +/- 0.00007Rs, consistent with evolutionary models. The tidally
distorted M star has a mass of 0.382 +/- 0.006Ms and a radius of 0.381 +/-
0.003Rs, also consistent with evolutionary models. We find that the magnesium
absorption line from the white dwarf is broader than expected. This could be
due to rotation (implying a spin period of only ~700 seconds), or due to a weak
(~100kG) magnetic field, we favour the latter interpretation. Since the M
star's radius is still within its Roche lobe and there is no evidence that its
over-inflated we conclude that QS Vir is most likely a pre-cataclysmic binary
just about to become semi-detached.
