Properties and nature of Be stars XIX. Spectral and light variability of 60 Cygni

An analysis of electronic spectra secured between 1992 and 1999 at the Haut e Provence, Ondrejov and Dominion Astrophysical Observatories and of differ ential UBV measurements of 60 Cyg obtained between 1984 and 1999 at Hvar, S an Pedro Martir, Toronto and Xinglong Observatories, the all-sky Hipparcos satellite H-p photometry transformed to Johnson V and B magnitudes, and all -sky UBV observations published by several authors and dating back to fifti es, led to the following findings: 1. 60 Cyg exhibits pronounced long-term spectral variations characterized b y the B --> Be --> B phase transitions. These long-term spectral changes of 60 Cyg are also accompanied by corresponding, though rather mild, secular light and colour variations. The character of these variations is indicativ e of a positive correlation between the brightness and emission-line streng th. 2. NLTE model atmosphere analysis of spectra secured during the quiescence state (B phase) of 60 Cyg shows that the star has overabundance of helium. Best results were obtained for N-He/N-H = 0.2. 3. The presence of periodic medium-term changes, with a period of 146.(d)6 +/- 0.(d)6 was found in the radial-velocity of the H alpha and He I 6678 An gstrom lines. If confirmed by future observations, these variations could i ndicate that 60 Cyg is a spectroscopic binary. 4. There are clear rapid periodic line-profile changes of (a) overall line asymmetry, and (b) weak sub-features passing across the line profiles every about 0.(d)1. The radial velocity and asymmetry of He I lines vary with a period of 1.(d)0647 and a double-wave curve. There is no evidence of this p eriod in photometry, however. 5. The rapid light variations of 60 Cyg are dominated by rapid changes with a full amplitude of almost 0.(m)1. A period analysis of V magnitude data p rewhitened for the long-term changes indicates a period of 0.(d)2997029, re ported earlier. The most interesting finding is that also ail recorded seri es of moving sub-features in the line profiles can be reconciled with this period: the sub-features reappear at the same phase intervals of the 0.(d)2 997 period in the line profiles over an interval of several years. Consider ing the acceleration of these sub-features, 1900 km s(-1)d(-1), it is conce ivable that the true physical (super) period of these changes is either 0.( d)8991 or 1.(d)1988. 6. The findings mentioned in points 4 and 5 represent a challenge for the N RP scenario since the light changes would be dominated by a high-order mode instead of a low-order one.