The optically thin dust shell of Nova Cygni 1978

Nova Cygni 1978 was monitored photometrically from V to 19.5 microns for 120 days after the eruption. Following the initial expansion of the hot gas shell, an optically thin dust shell formed and reached a maximum visual optical depth of about 0.1 by day 60. No visible transition phase of the type observed in the very dusty DQ Herculis novae occurred in Nova Cygni 1978. It is argued that dust grain growth was inhibited because of the low mass of condensable atoms in the shell. Although the dust shell in Nova Cygni 1978 was optically thin at visual and infrared wavelengths, the grains grew to a radius of 0.3 micron which is comparable to the radius of the grains which grow in the optically thick shells of the DQ Herculis type novae.