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We present {\sl Hubble Space Telescope\/} observations of the peculiar Seyfert~1 galaxy Mrk~231. Ultraviolet (1600--3300~\AA ) linear spectropolarimetric measurements of the object's nuclear region were obtained using the {\sl Faint Object Spectrograph\/}. New ground-based, high signal-to-noise ratio optical spectropolarimetry is also presented. Both the degree and position angle of the UV continuum polarization redward of the Mg~II emission line are comparable to the values observed at the shortest wavelengths accessible to ground-based instruments ($P \sim 15$\%; $\theta \sim 90^{\circ}$). However, from the peak of the redshifted Mg~II emission line through its P-Cygni-like absorption feature, the polarization is depressed to half that level (similar to the decrease in $P\/$ seen in the blue wing of H$\alpha$). By $\sim$2700~\AA\ (observed frame) the polarization recovers to about 17\% but then falls rapidly at shorter wavelengths. In fact, for $\lambda < 1800$~\AA\ the object is essentially unpolarized.
The complex wavelength dependence of the UV polarization is evidence that light from a young stellar population (presumably located around the active nucleus) dilutes the polarization from the highly reddened AGN. Unreddened starlight that dominates the flux at wavelengths shorter than about 2400~\AA\ also explains the abrupt change in the spectral slope seen in this part of the spectrum. Decomposition of the AGN and stellar components to the total flux indicates that the intrinsic polarization of the AGN light exceeds 20\% in the UV. In addition, evidence from optical polarimetry that there is more than one scattering cloud producing the observed polarization in Mrk~231 is supported by the fact that $\theta\/$ rotates from 90$^{\circ}$ to 130$^{\circ}$ between 3300 and 1800~\AA .
This research is supported by NASA grant NAG~5--1630 and NSF grant AST~91--14087.
