2 The nebula

The HST/WFPC2 appearance of the high excitation PN NGC 6818 (sometimes called the Little Gem) is shown in Fig. 1: in [O III] "a roughly spherical outer envelope as well as a brighter vase-shaped interior bubble. There is a prominent orifice to the North and a smaller one to the South, along the major axis, probably caused by a blow-out from a fast wind'' (Rubin et al. 1998). A few dark filaments and knots are also present.

The [N II] emission mainly occurs in two irregular equatorial "moustaches'' and in a multitude of condensations (in some cases presenting a sort of radially arranged cometary tail), which are brighter in the southern part of the nebula.

The striking HST multicolor reproduction by Arsen Hajian & Yervant Terzian (http://ad.usno.navy.mil/pne/gallery.html) highlights the composite envelope structure and the large stratification of the radiation within NGC 6818.

Rubin et al. (1998) reported the presence of two faint stars, roughly 2-4 arcsec N and NE of the central star, which could be physically associated to it. A careful analysis of the WFPC2 frames allowed us to unveil the true binary nature of the central star: the red, faint companion is at a distance of 0.093($\pm$0.004) arcsec in position angle (PA) $= 190(\pm 2)\hbox{$^\circ$ }$ (see the enlargement at the bottom of Fig. 1).

Following Weedman (1968) our nebula is a prolate spheroid (an ellipse rotated about the major axis) of moderate ellipticity (axial ratio 3:2) observed perpendicular to the major axis, whereas Sabbadin (1984) suggests a somewhat optically thick prolate spheroid in a early evolutionary phase, seen almost pole-on. According to Hyung et al. (1999), a definitive statement on the geometry must wait for more adequate kinematical data, due to the complexity of NGC 6818.

To this end we have secured a series of long-slit, radially arranged echellograms. They were analysed with the 3-D methodology described in Papers I to IV.