  Planetary nebulae are ionized clouds of gas formed by the hydrogen-rich
envelopes of low- and intermediate-mass stars ejected at late evolutionary
stages. The strong UV flux from their central stars causes a highly stratified
ionization structure, with species of higher ionization potential closer to the
star. Here we report on the exceptional case of HuBi 1, a double-shell
planetary nebula whose inner shell presents emission from low-ionization
species close to the star and emission from high-ionization species farther
away. Spectral analysis demonstrates that the inner shell of HuBi 1 is excited
by shocks, whereas its outer shell is recombining. The anomalous excitation of
these shells can be traced to its low-temperature [WC10] central star whose
optical brightness has declined continuously by 10 magnitudes in a period of 46
years. Evolutionary models reveal that this star is the descendent of a
low-mass star ($\simeq$1.1 $M_\odot$) that has experienced a born-again event
whose ejecta shock-excite the inner shell. HuBi 1 represents the missing link
in the formation of metal-rich central stars of planetary nebulae from low-mass
progenitors, offering unique insight regarding the future evolution of the
born-again Sakurai's object. Coming from a solar-mass progenitor, HuBi 1
represents a potential end-state for our Sun.
