Epimetheus (or S11 Epimetheus) and Janus are co-orbital
    satellites of Saturn; they exchange orbits every four years. The
    following properties are from K.R. Lang, 'Astrophysical Data:
    Planets and Stars', p.71:
 
                                      Janus       Epimetheus
      Distance from planet center:     151.4         151.5     km
                                         2.510         2.511   planet radii
                   Orbital period:       0.694         0.695   days
                     Eccentricity:       0.009         0.007   degrees
                      Inclination:       0.3           0.1     degrees