  The isomorphism problem is known to be efficiently solvable for interval
graphs, while for the larger class of circular-arc graphs its complexity status
stays open. We consider the intermediate class of intersection graphs for
families of circular arcs that satisfy the Helly property. We solve the
isomorphism problem for this class in logarithmic space. If an input graph has
a Helly circular-arc model, our algorithm constructs it canonically, which
means that the models constructed for isomorphic graphs are equal.
