eigenvalues with zero or closest to zero imaginary parts?

I am using eigs(A,B,k,sigma) to calculate a complex Eigen value problem. A and B are both large, sparse complex matrices.

I am only interested in eigenvalues that has an imaginary part closest to zero. However, if I set sigma='si', then I got eigenvalues with a negative imaginary part, rather than a zero imaginary part. How to get eigenvalues that have zero or closest to zero imaginary parts please?