1996A&A...312..121T

Result. based on the 30 best nightly runs are reported. The moments of 33 brightness maxima and 35 minima show large scatter of phases within 0.6^p-interval. The value of the primary photometric period of P=0.132953±13x10^–6d differs from the previously published ones. Periodograms for detrended nightly runs show significant peaks at different frequencies from 28 to 103c/d. The most prominent peaks correspond to periods of 23.7, 15.2, 27.5 and 51.6min. This argues for transient quasi-periodic oscillations (QPO) occurring at few preferred time scales rather than for a QPO with a time scale decreasing from 27min in 1962 to 17min in 1985 (Semeniuk et al. 1987). Such behaviour may be explained by the "beat-frequency model" (Hollander & van Paradijs 1992). It seems that in TT Ari we observe contributions of several instability mechanisms with similar time scales. At higher frequencies 90-900 c/d the periodogram for all observations obeys a power law with an index γ=1.600±0.008. This value coincides within error estimates with that of γ=1.51±0.04 obtained for a mean periodogram. Such apparent red noise may be owed to the QPOs. The variations in UBVRI are highly correlated. A method for determination of relative colours of "slow", "fast correlated" variations and an "uncorrelated noise" is proposed. Corresponding parameters are determined. Values U-B=-1.04mag, -1.24mag, -1.35mag and B-V=0.19mag, 0.05mag, 0.12mag for "mean brightness", "fast" and "slow" variations, respectively.