A homogeneous sample of 24 elliptical multiple-shell planetary nebulae (EM PNs) have been studied together with their central stars in order to disclose possible correlations between the stellar and nebular aspects and to find the evolutionary paths yielding to these observed configurations. We propose a finer classification for EM PNs, depending on the appearance of the outer halo, based on detailed Hα surface brightness maps. We define detached-halo (DH), attached-halo (AH), and attached-halo/irregulars (AH/I) PNs. The evolutionary paths examined in comparison to the observations were (1) multiple ejection at the thermal pulsating asymptotic giant branch phase (TP-AGB), including wind and superwind; (2) shell ejection during the postasymptotic giant branch (post-AGB) cooling phase; and (3) asymmetric shell expansion. Our analysis indicates that 1. DH and AH PNs have different intrinsic nature, they show distinct galactic distributions, and they seem to originate through different evolutionary paths. DH PNs are round, while AH PNs often show marked ellipticity. 2. DH PNs might originate from multiple shell-ejection at the TP-AGB or from a "born again" episode, which is indicated by the spectral type of the central star. 3. Most AH PNs owe their shapes to dynamical processes due to winds interaction, mostly asymmetrical. 4. One AH/I and two AH PNs are probably projected bipolars, as their chemistry suggests.
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Stanghellini et al. (1995) studied this question.