1 Introduction

A source of extended radio emission in the Galaxy is usually considered to be a candidate supernova remnant if the radio spectrum is non-thermal, as its origin may be synchrotron emission. Optical and/or X-ray observations can contribute to its positive identification as a remnant, provided that emission is detected and is not absorbed by the interstellar medium. At optical wavelengths significant attenuation due to interstellar extinction allows only relatively nearby remnants to be observable. It is well known that the interstellar medium is far from uniform and homogeneous. Cavities can be found where the density is much lower than the ambient density, as well as regions of much higher density, known as interstellar "clouds''. Optical emission from middle-aged remnants is produced by the interaction of the main blast wave with these interstellar clouds. Observations with optical telescopes through narrow-band interference filters e.g. H$\alpha$, [S II], [O III], and/or long-slit spectra can provide us with information about the shock speed, the electron density and temperature and possible interactions with the local interstellar medium (e.g. Osterbrock 1989; Cox & Raymond 1985). Wide field imaging offers the additional advantage of studying the morphogology in each emission line and searching for correlations with emission at radio, X-ray and infrared wavelengths.

During deep optical observations of the supernova remnant CTB 80 by Mavromatakis et al. (2001, their Fig. 1), two faint, thin arcs were discovered at the north-east edge of the field of view. Their extent, shape and faintness motivated us to perform new deep observations of the area to the east of the two arcs. In this paper we present deep CCD images of this field in major optical lines. Information about the observations and the data reduction is given in Sect. 2. In Sects. 3 and 4 we present the results of our imaging observations as well as results from long-slit spectra taken at specific locations of interest. In Sect. 5 we report on the results of a search for emission in radio, X-ray and far-infrared wavelengths. Finally, in Sect. 6 we discuss the physical properties of the structures discovered by us.

Figure 1: The field around the candidate remnant in the H$\alpha +[$N II] filter. The image has been smoothed to suppress the residuals from the imperfect continuum subtraction. Shadings run linearly from 0 to 80 $\times$ 10-17 erg s-1 cm-2 arcsec-2. The line segments seen near overexposed stars in this figure and the next figures are due to the blooming effect.