  A mixed graph is a graph with some directed edges and some undirected edges.
We introduce the notion of mixed matroids as a generalization of mixed graphs.
A mixed matroid can be viewed as an oriented matroid in which the signs over a
fixed subset of the ground set have been forgotten. We extend to mixed matroids
standard definitions from oriented matroids, establish basic properties, and
study questions regarding the reorientations of the unsigned elements. In
particular we address in the context of mixed matroids the P-connectivity and
P-orientability issues which have been recently introduced for mixed graphs.
