Left: composite of the M 17 mosaic obtained with the IAC80 telescope. In the color figure reproduced in the electronic version of the journal, blue corresponds to emission in the [OIII] filter, green in the Hα filter, and red in the [SII] filter. The area covered is , with north to the top and east to the left. The yellow square indicates the area enlarged in the panels on the right, which contains the structures described in this paper, including three emission knots near the upper right corner and the bright arc-shaped nebula near the bottom left. For reference, an image in the narrow-band continuum adjacent to Hα is included at the bottom right showing stellar sources in the field. Both the knots and the arc are best seen in Fig. 2.