Since PN morphology is presumably connected with the evolution of the ensemble PN - central star, available observations obviously show a large variety of shapes, which could reflect different stages of this evolution. Progress in theories (Frank et al. 1993; Bryce et al. 1994) requires a large, homogeneous set of monochromatic images and isophotes (Kaler 1985) which narrow bandpass CCD imaging of PN can supply (Balick 1987).
Here we present new sensitive observations using selective filters for seven
galactic type I PN (Peimbert 1978; Peimbert & Serano
1980) which are characterized by high helium and nitrogen contents
and which are believed to originate from massive precursor stars (Peimbert
& Peimbert 1983). Our goal in this study was to search for the
so-called secondary structures, if any, around the bright central nebula,
and which could be relics of primary ejections by the evolving ionizing
star in the AGB phase. The overall diameter measured on the
monochromatic images should allow new estimates of the ionized mass of
the planetary nebula. As a matter of fact, existing inventories of this
parameter in PN reveal a shortfall as compared with theory.