Introduction

VV 523 (= NGC 3991 – UGC 6933 – IRAS11549+3237) is a typical clumpy irregular galaxy. Its distance is 42.6 Mpc, its absolute blue magnitude is −20.2 and its U – B index –0.44. On CCD frames taken at the Bernard Lyot 2-m telescope of the Observatoire du Pic-du- Midi VV 523 shows a complex structure with three components of bright clumps scattered in a common envelope. The extension of the envelope is 80″ × 2″ within a surface brightness of 25 mag arcsec−2.

Observations

Three filters (B, R and I) were used for the CCD observations. The seeing was about 1 arcsecond. A wavelet transform with a scale parameter suited to the size of the starforming cells was used to detect the clumps and to provide their size, location and colour indexes. Figure 1 shows the wavelet transform of the B image. The photometric results are in Table 1. The clumps have a mean size of 200 parsecs and the B – I indexes exhibit a sequence in colour which corresponds to their location in the galaxy.

Long-slit spectra with a dispersion of 33 and 260 Å mm−1 were recorded with the Carelec spectrograph at the 1.93-m telescope of the Observatoire de Haute Provence. The spectra show strong emission lines typical of HII regions. Three regions of different intensities are detected and named A, B and C. In Table 1 we give the physical properties of each of the three regions.