We analyze optical images and high-resolution, long-slit spectra of three p
lanetary nebulae that possess collimated, low-ionization features. NGC 3918
is composed of an inner, spindle-shaped shell mildly inclined with respect
to the plane of the sky. Departing from the polar regions of this shell, w
e find a two-sided jet expanding with velocities that increase linearly wit
h distance from 50 to 100 km s(-1) The jet is probably coeval with the inne
r shell (age approximate to 10000 yr, where D is the distance in kpc), sugg
esting that its formation should be ascribed to the same dynamical processe
s that also shaped the main nebula, and not to a more recent mass-loss epis
ode. We discuss the formation of the aspherical shell and jet in the light
of current hydrodynamical and magnetohydrodynamical theories. K1-2 is a pla
netary nebula with a close binary nucleus that shows a collimated string of
knots embedded in a diffuse, elliptical shell. The knots expand with a vel
ocity similar to that of the elliptical nebula (similar to 25 km s(-1)), ex
cept for an extended tail located out of the main nebula, which linearly ac
celerates up to similar to 45 km s(-1). We estimate an inclination on the l
ine of the sight of similar to 40 degrees for the string of knots; once the
orientation of the orbit is also determined, this information will allow u
s to test the prediction of current theories of the occurrence of polar jet
s from close binary systems. Wray 17-1 has a complex morphology, showing tw
o pairs of low-ionization structures located in almost perpendicular direct
ions from the central star, and embedded in a large, diffuse nebula. The tw
o pairs show notable similarities and differences, and their origin is very
puzzling.