  We report the results of Faraday rotation measurements of 23 background radio
sources whose lines of sight pass through or close to the Rosette Nebula. The
Rosette Nebula is an excellent candidate for studies of super bubbles
associated with young star clusters. We made linear polarization measurements
with the Karl G. Jansky Very Large Array (JVLA) at frequencies of 4.4GHz,
4.9GHz, and 7.7GHz. We are able to establish a background rotation measure in
this part of the sky due to the Galaxy of +147 rad m^-2. Sources whose lines of
sight pass through the nebula have an excess rotation measure of 50-750 rad
m^-2, which we attribute to the plasma shell of the Rosette Nebula. We consider
two simple plasma shell models and how they reproduce the magnitude and sign of
the rotation measure, and its dependence on distance from the center of the
nebula. These two models represent different modes of interaction of the
Rosette Nebula star cluster with the surrounding interstellar medium. Both can
reproduce the magnitude and spatial extent of the rotation measure enhancement,
given plausible free parameters. We contend that the model based on a stellar
bubble more closely reproduces the observed dependence of rotation measure on
distance from the center of the nebula.
