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Summary form only given. A recent theoretical analysis has shown that diffractive elements, like apertures, can interact with the laser light polarization and change the symmetry and, hence, the pattern structure of the laser. However, there are many possible causes for symmetry breaking in a laser, one of them being the astigmatism induced by the Brewster windows. A great variety of complex patterns have been experimentally observed in a CO/sub 2/ laser containing Brewster windows and an iris. Many of these patterns seem to be compatible with a reduced symmetry of the laser. In our experiments, this reduction is mainly due to astigmatism rather than polarization effects.