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tokens: in the strong peaks of emission towards the top and bottom of this figure, at distance of 19 arcsec or 1.8 kpc from the nucleus. The spectrum shows complex nuclear structure, with multiple components, including a very rapidly rotating structure, which we interpret as a star-forming nuclear ring. Between the nuclear and bar-end rings, diffuse emission is apparent, particularly in the upper part of Fig. A2 . Thus, in spite of its peculiarity, we do consider this an example of the SFD phenomen, and it is included as such in this paper. Figure A2. Two-dimensional optical spectrum of NGC 3729, centred on the H α and [N ii ] lines. The emission-line clump is indicated by the arrows. Comparison with the bright H ii region towards the bottom of the frame shows the extended nature of the line emission from the clump. The unusual feature seen in Fig. A2 is the peak of emission seen just below the nuclear ring, 5.7 arcsec (560 pc) from the nucleus. At first sight, this resembles an H ii region, but uniquely, in the current data set at least, it shows clear LINER-type ratios. To illustrate this point, Fig. A3 shows the extracted spectrum of this peak in the lower frame, with an H ii region spectrum from the same long-slit observation shown in the upper frame for comparison. The spectra are clearly different, and argue against ongoing SF in the near-nuclear clump. The clump has [N ii ] and H α EW values of 5.63 and 6.73 Å, respectively, similar to or slightly greater than the values for the two SFD regions in NGC 3729 (4.70 and 3.69 Å for [N ii ], 6.76 and 4.50 Å for H α). These values are all at the high end of the EW distributions for line emission from SFD regions, while the [N ii ]/Hα ratio for the clump, 0.837, is entirely consistent with the distribution of ratios for SFD regions (Fig. 1 , lower panel). Figure A3. Extracted H α and [N ii ] emission for the NGC 3729 clump (bottom) and, for comparison, an H ii region in the same galaxy (top). The high [N ii ]/H α ratio in the clump is clearly apparent. One possibility is that the near-nuclear clump could be the nucleus of the tidally shredded companion that has been drawn towards the centre of NGC 3729 through dynamical friction. One test of this is to look for spatial extent. Visual inspection of Fig. A2 indicates that the clump appears moderately extended, e.g. compared with the most compact H ii region (towards the bottom of the image). Measurements confirm this; along the spatial direction, the clump has a full width at half-maximum (FWHM) of 5.3 pixels, or 2.33 arcsec, compared to 3.7 pixels or 1.63 arcsec for the compact H ii region. As the latter may itself be somewhat extended, we also measured the FWHM of the standard star observation taken closest in time to the NGC 3729 spectra, finding values of 3.2 pixels or 1.4 arcsec. Taking either the star or (conservatively) the H ii region as a measure of the seeing broadening the clump FWHM, the true size of the clump is 1.9 or 1.7 arcsec,