def _B_at_h(h1, h2):
    """Evaluates B auxiliary variable at given h coefficients.

    Parameters
    ----------
    h1: float
        The first of the h coefficients.
    h2: float
        The second of the h coefficients.

    Returns
    -------
    B: float
        Auxiliary variable.

    Notes
    -----
    This is equation (56) from the original report.

    """
    B = (27 * h2) / (4 * (1 + h1) ** 3)
    return B