def approximate_response_function(f, fstar):
    """Approximate LISA response function

    Use Eq.9 of Robson+19 to approximate the LISA response function.

    Parameters
    ----------
    f : `float/array`
        Frequencies at which to evaluate the sensitivity curve

    fstar : `float`
        f* from Robson+19 (default = 19.09 mHz)

    Returns
    -------
    R : `float/array`
        response function at each frequency
    """
    return (3 / 10) / (1 + 0.6 * (f / fstar)**2)