into molecular gas of $n_{\rm H}=10^4$ cm$^{-3}$ according to the $C$-shock model of [@dra83]. We were unable to find model calculations for lower densities. But, since the intensity will be proportional to the preshock density, provided that the density in the emitting gas is less than the critical density [$\simgt 10^5$ cm$^{-3}$; @bur89], the results of [@dra83] indicates that a 40–50  shock propagating into molecular gas of $n_{\rm H}=10^3$ cm$^{-3}$ might have similar (normal) surface brightness. A slower shock with a lower preshock density would be possible if the shock propagating into the filament is tangential along the line of sight, so that the brightness normal to the shock front is lower.

The situation is not so clear for the -NE filament for which we lack spectroscopic information. Its flux density, however, is comparable to that of the SE filament and we may rule out the excitation by X-rays from 11p2. We checked 2MASS colors of nearby ($\le 2'$) stars, but found no OB stars that would be responsible for the UV excitation. This leaves again the shock excitation for the origin of the emission. A difficulty with the shock excitation is that the filament is located outside the radio 