Through a program of narrow band imaging , we have observed the changing structure of the H \alpha emission line around Nova Cyg 1992 ( V1974 Cyg ) at regular intervals from 1996 to 1999 .
Between 1994 and 1996 , the nebular boundary advanced to the southwest at nearly the speed of light , implying that the nebula was created by an expanding wave of radiation originating in the explosion interacting with surrounding material .
The expansion speed dropped to 0.35 c during 1996-1999 .
We have taken spectra of the nebula in 1998 and 1999 .
Only Balmer lines are detected , no He I , [ O III ] , [ O II ] , [ N II ] , or [ S II ] .
There is also no trace of the high excitation nova lines ( He II , NeV , etc ) .
The Balmer lines are unresolved in velocity ( FWHM \leq 100 km s ^ { -1 } ) .
These spectra show that the nebula is not a reflection nebula , a conventional H II region , or a shock involving motions of the gas .
The integrated H \alpha luminosity of the nebula between 1996 and 1999 is in the range \simeq 1.3-2.2 \times 10 ^ { 35 } erg s ^ { -1 } .

The Balmer decrement is normal for recombinations of a lightly reddened plasma .
The lack of forbidden emission lines can only be understood if the electron temperature is low .
This condition results if the energies of the ejected photoelectrons are shared among electrons , protons , and neutrals in a partially ionized medium .
The He I lines are suppressed if the flash ionizing spectrum is truncated at or below the He ^ { 0 } ionization edge .
The ionized material is on the front face of neutral sheets .
The density is poorly determined , but is probably very large ( \sim 10 ^ { 4 } cm ^ { -3 } ) in order to explain the brightest region of the nebula .
The dynamical timescale is about a year and the recombination timescale of the same order .
Bright patches are observed to fade in these times .
The energy required to ionize the nebula is the bolometric luminosity of the nova for 30 days , smaller than the time during which the temperature of the nova photosphere was in the right range to produce the ionizing photons .

We have also undertaken sensitive surveys of H \alpha nebulae around recent novae but find no evidence of other such nebulae , so this type of object must be rare .