We have presented deep monochromatic images of seven type I galactic planetary
nebulae. Except for the case of A 39 (where we have not detected outer
emission) the observations evidence bipolarity and multiple-shell structures.
The linear diameters generally exceed 1 pc. Consequently, the derived ionized
mass is often larger than 
. This might contribute to the
solution of the mass deficit in planetary nebulae.
Acknowledgements
It is our great pleasure to thank INSU and the G0968 GDR "Milieux Circumstellaires" for travel grant, and our colleagues of the University of Canterbury (Drs. J.B. Hearnshaw, W. Tobin) for their friendly advices. Our thanks also go to Mike Clark for his assistance during the observations at the MJUO telescope. We are indebted to Prof. Michael Dopita of Mt Stromlo for fruitful discussions and corrections to improve the manuscript, to Drs. J. Lequeux, J.M. Deharveng and to the anonymous referee for pertinent comments.