def rKerr_LR_Bardeen(j, M=1, branch=0):
    """

    Calculate Kerr Light-Ring Radius

    USAGE:
    rKerr_LR_Bardeen(Dimensionless_BH_Spin,BH_Mass)

    ~ londonl@mit.edu

    """
    from numpy import cos, arccos, sqrt
    from positive import acos
    rKerr_LR = 2 * M * (1 + cos(2.0 / 3 * acos(-j, branch=branch)))
    return rKerr_LR
