def retr_smaxkepl(peri, masstotl):
    
    """
    Get the semi-major axis of a Keplerian orbit (in AU) from the orbital period (in days) and total mass (in Solar masses).

    Arguments
        peri: orbital period [days]
        masstotl: total mass of the system [Solar Masses]
    Returns
        smax: the semi-major axis of a Keplerian orbit [AU]
    """
    
    smax = (7.496e-6 * masstotl * peri**2)**(1. / 3.) # [AU]
    
    return smax