Infrared observations of Nova Vulpeculae 1976 (NQ Vul).

Broad-band infrared photometry was carried out for Nova Vulpeculae 1976 (NQ Vul) for the period covering two hundred days after its discovery. After a gradual decline parallel to that in the optical flux, the infrared radiation suddenly increased by about 3.5 mag at the L-band. The increase began on the 50th day after the explosion, in coincidence with an abrupt drop in the visible light. The transition happened very fast in 20 d, reaching the maximum at around 70th day. Then the infrared radiation faded slowly by 4 magnitudes in more than 150 d, while the radiation in visible region recovered slightly and kept a nearly constant level in the last 100 d. Dust formation and growth in the exploded shell are discussed in connection with the observed results in infrared and optical regions. Key words: Dust formation; Infrared photometry; Nova Vulpeculae 1976.