def u_to_q(u1, u2):
    """convert limb-darkening coefficients from q to u
    See Kipping 2013, eq. 15 & 16:
    https://arxiv.org/pdf/1311.1170v3.pdf
    """
    q1 = (u1 + u2) ** 2
    q2 = u1 / (2 * (u1 + u2))
    return q1, q2