Continuum variability in Nova Cygni 1975

ONE of the more unusual aspects of Nova Cygni 1975 (V1500 Cyg) has been the presence of nearly periodic (∼3.2 h) photometric variations beginning early in the decline (discovered by Tempesti¹ on September 9-10, 1975) and lasting up to the present. In spite of the excellent coverage available, the cause of this photometric activity is uncertain. In particular, broadband photometry has been unable to discriminate between continuum and emission line variations, which is necessary because the relative importance of continuum and line emission has changed as the nova has developed. The relationship of this variation to classical transition phase oscillations in novae is also uncertain. We have obtained very low noise (s/n ∼ 100) time resolved (∼ 11 min per observation, extending over 87 min) 0.8-Å resolution scans of the [OIII] λλ 4959, 5,007-Å complex in Nova Cygni starting at 0943 UT on August 21, 1976, using a multichannel Reticon (Vogt and Tull, in preparation) at the coudé focus of the 2.7-m telescope at the McDonald Observatory. The scans, which are shown in Fig. 1, establish the stability (profile variations < 5%) of this feature. We infer the constancy of the [OIII] λλ 4959, 5,007-Å emission feature and further infer the constancy of the other forbidden lines.