def compute_chi_eff(m1,m2,s1,s2):
    """ Compute chi effective spin parameter (for a given component)
        --------
        m1 = primary mass component [solar masses]
        m2 = secondary mass component [solar masses]
        s1 = primary spin z-component [dimensionless]
        s2 = secondary spin z-component [dimensionless]
    """
    return (m1*s1+m2*s2)/(m1+m2)