  We report the first unambiguous observational evidence of Rydberg Enhanced
Recombination (RER), a potentially important recombination mechanism that has
hitherto been unexplored in low-temperature photoionized plasmas. RER shares
similarities with dielectronic recombination, with the difference that the
electron is captured into a highly excited state below the ionization threshold
-- rather than above the threshold -- of the recombining ion. We predict
transitions of carbon and oxygen ions that are formed via the RER process, and
their relative strengths with collisional-radiative spectral models. Optical C
II RER features are detected in published high-resolution spectra of eight
planetary nebulae, and a C III transition has been found in the ultraviolet
spectrum in a symbiotic star system. The relative intensities of these lines
are consistent with their production by this recombination mechanism. Because
RER has not previously been accounted for in photoionized plasmas, its
inclusion in models can significantly impact the predicted ionization balance
and hence abundance calculations of important astrophysical species.
Calculations for C+ suggest that the enhancement in the total recombination
rate can amount to a factor of 2.2 at 8100 K, increasing to 7.5 at T = 3500 K.
These results demonstrate the importance of including RER in models of
photoionized astrophysical plasmas and in elemental abundance determinations.
