def p_from_a(sep, m1, m2):
    """ Computes separation from orbital period with kepler III

    Parameters
    ----------
    sep : float/array
        separation [rsun]
    m1 : float/array
        primary mass [msun]
    m2 : float/array
        secondary mass [msun]

    Returns
    -------
    p : float/array
        orbital period [day]
    """
    sep_au = sep / 215.032
    p_2 = sep_au ** 3 / (m1 + m2)
    p_day = p_2 ** 0.5 * 365.25
    return p_day
