  We present MERLIN observations of OH maser and radio continuum emission from
the Ultra Luminous IR Galaxy Markarian 231. The 1665- and 1667-MHz transitions
have a combined velocity extent of 720 km/s and show a similar
position-velocity structure including a gradient of 1.7 km/s/pc from NW to SE
along the 420-pc major axis, steeper in the inner few tens of pc. The maser
distribution is modelled as a torus rotating about an axis inclined at ~45deg.
We estimate the enclosed mass density to be 320(90) Msun in a flattened
distribution, including a central unresolved mass of </=8E+06 Msun. All the
maser emission is projected against a region with a radio continuum brightness
temperature >/=1E+05 K, giving a maser gain of </=2.2. The 1667:1665-MHz line
ratio is close to the LTE ratio of 1.8 consistent with radiatively pumped,
unsaturated masers. The size of individual masing regions is in the range
0.25-4 pc with a covering factor close to unity. There are no very bright
compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273
where the masing torus is viewed nearer edge-on. The comparatively modest maser
amplification seen from Markarian 231 is consistent with its classification as
a Seyfert 1. Most of the radio continuum emission on 50-500 pc scales is
probably of starburst origin but the compact peak is 0.4 per cent polarized by
a magnetic field running north-south, similar to the jet direction on these
scales. There is no close correlation between maser and continuum intensity.
Comparisons with other data show that the jet changes direction close the
nucleus and suggest that the sub-kpc disc hosting the masers and starburst
activity is severely warped.
