Title:Detection of resonant orbits in the spiral structure of the Galaxy  traced by CS sources and by 
Cepheids.
 

Abstract:
We discuss the interpretation of a  map of the spiral structure of the Galaxy traced by molecular CS 
emission associated with IRAS sources (the catalog of Bronfmann et al.), which give a better description of 
the overall spiral structure than the HII regions. The well defined spiral arms are seen to be confined inside 
the corotation radius, while the arms outside corotation are broader, as is often the case in spiral galaxies. 
We identify a square-shaped sub-structure in the CS map with the predicted one associated with stellar 
orbits at the 4:1 resonance (4 epicycle oscillations in one turn around the galactic center). We argue that 
such structures are seen in other galaxies too.
The sub-structure is found at the expected radius, based on the known pattern rotation speed and epicycle 
frequency curve.  It is also in agreement with the position of the ring-shaped gap in the HI distribution that 
we recently detected (a few words will also be given to explain the nature  of this gap). As additional results, 
an inner arm presents an end with strong inward curvature and intense star formation that we tentatively 
associate with the region where this arm surrounds the extremity of the bar, as seen in many barred 
galaxies. Finally, a new arm with concave curvature is found in the Sagitta to Cepheus region of the sky.