Infrared emission by dust in NGC 1068 and three planetary nebulae.

Photometric observations of NGC 1068, NGC 7027, IC 418, and BD + 3003639 between 5 and 27 are reported. The observations are explained as thermal radiation from a form of dust that shows spectral features, but which has not yet been identified, and resembles that in the carbon-rich RV Tauri star AC Her. Arguments are deduced for its being a condensate from a carbon- and nitrogen-rich gas in an excited environment. Subject headings: infrared sources - planetary nebulae - Seyfert galaxies