def Gamma1_gasrad(beta):
    """
    Gamma1 for a mix of ideal gas and radiation

    Hansen & Kawaler, page 177, Eqn. 3.110

    Parameters
    ----------
    beta : float
        Gas pressure fraction Pgas/(Pgas+Prad)

    """
    Gamma3minus1 = (2./3.) * (4. - (3.*beta)) / (8. - (7.*beta))
    Gamma1 = beta + (4. - (3.*beta)) * Gamma3minus1
    return Gamma1