  Helmet streamers are a prominent manifestation of magnetic structures with
current sheets in the solar corona. These large-scale structures are regions
with high plasma density, overlying active regions and filament channels. We
investigate the three-dimensional (3D) structure of a coronal streamer,
observed simultaneously by white-light coronagraphs from two vantage points
near quadrature (SOHO/LASCO and STEREO/COR2). We design a forward model based
on plausible assumptions about the 3D streamer structure taken from physical
models (a plasma slab centered around a current sheet). The streamer stalk is
approximated by a plasma slab, with electron density that is characterized by
three separate functions describing the radial, transverse and face-on profiles
respectively. For the first time, we simultaneously fit the observational data
from SOHO and STEREO using a multivariate minimization algorithm. The streamer
plasma sheet contains a number of brighter and darker ray-like structures with
the density contrast up to about a factor 3 between them. The densities derived
using polarized and unpolarized data are similar. We demonstrate that our model
corresponds well to the observations.
