Diamond-free circle graphs are Helly circle

The diamond is the graph obtained from K 4 by deleting an edge. Circle graphs are the intersection graphs of chords in a circle. Such a circle model has the Helly property if every three pairwise intersecting chords intersect in a single point, and a graph is Helly circle if it has a circle model with the Helly property. We show that the Helly circle graphs are the diamond-free circle graphs, as conjectured by Durلn. This characterization gives an efficient recognition algorithm for Helly circle graphs.