  We have analysed deep optical spectra of the `born-again' planetary nebula
Abell 58 and its hydrogen-deficient knot, surrounding V605 Aql, which underwent
a nova-like eruption in 1919. The electron temperature we derive for the
central knot varies widely depending on the diagnostic used. The [O III]
nebular-to-auroral transition ratio gives a temperature of 20800 K, while the
ratio of the [N II] nebular and auroral lines give Te=15200 K. The helium line
ratios 5876/4471 and 6678/4471 imply temperatures of 350 K and 550 K
respectively. Weakly temperature-sensitive O II recombination line ratios imply
similarly low electron temperatures. Abundances derived from recombination
lines are vastly higher than those found from collisionally excited lines, with
the abundance discrepancy factor (adf) for O2+ reaching 89 -- the second
highest known value after that found for the hydrogen deficient knots in Abell
30. The observed temperature diagnostics and abundances support the idea that,
like Abell 30, the knot of Abell 58 contains some very cold ionised material.
Although the central star is carbon-rich (C/O>1), the knot is found to be
oxygen-rich, a situation not predicted by the single-star `born again' theory
of its formation. We argue that the abundances in the ejecta observed in A 30
and A 58 have more in common with neon novae than with Sakurai's Object, which
is believed to have undergone a final helium flash. In particular, the C/O
ratio of less than unity and presence of substantial quantities of neon in the
ejecta of both Abell 30 and Abell 58 are not predicted by very late thermal
pulse models.
