The spectrum of R Cygni during its exceptionally low maximum of 1983.

In 1983 R Cygni experienced its faintest maximum ever recorded. A study of the light curve shows correlations between brightness at maximum and interval from the previous cycle, in the sense that fainter maxima occur later than normal and are followed by maxima that occur earlier than normal. Emission and absorption lines in the optical and near infrared (2.2 μm region) reveal two significant correlations. The amplitude of line doubling is independent of the magnitude at maximum for m_v(max)=7.1 to 9.8. The velocities of the emission lines, however, correlate with the magnitude at maximum, in that during bright maxima they are negatively displaced by 15 km s^-1 with respect to the red component of absorption lines, while during the faintest maximum there is no displacement. While this may be due to a reduced velocity of expansion during a faint maximum, we give reasons for interpreting it as an enhanced degree of limb brightening.