  The thickness of freshly made soap films is usually in the micron range, and
interference colors make thickness fluctuations easily visible. Circular
patterns of constant thickness are commonly observed, either a thin film disc
in a thicker film or the reverse. In this Letter, we evidence the line tension
at the origin of these circular patterns. Using a well controlled soap film
preparation, we produce a piece of thin film surrounded by a thicker film. The
thickness profile, measured with a spectral camera, leads to a line tension of
the order of 0.1 nN which drives the relaxation of the thin film shape,
initially very elongated, toward a circular shape.A balance between line
tension and air friction leads to a quantitative prediction of the relaxation
process. Such a line tension is expected to play a role in the production of
marginal regeneration patches, involved in soap film drainage and stability.
