PERIODIC AND APERIODIC VARIATIONS IN TT ARIETIS - RESULTS FROM AN INTERNATIONAL CAMPAIGN

Results based on the 30 best nightly runs are reported. The moments of 33 brightness maxima and 35 minima show large scatter of phases withi n 0.6(p)-interval. The value of the primary photometric period of P = 0.132953(d) +/- 13 . 10(-6) differs from the previously published ones . Periodograms for detrended nightly runs show significant peaks at di fferent frequencies from 28 to 103 c/d. The most prominent peaks corre spond to periods of 23.7, 15.2, 27.5 and 51.6 min. This argues for tra nsient quasi-periodic oscillations (QPO) occurring at few preferred ti me scales rather than for a QPO with a time scale decreasing from 27 m in in 1962 to 17 min in 1985 (Semeniuk et al. 1987). Such behaviour ma y be explained by the ''beat-frequency model'' (Hollander & van Paradi js 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 wit h an index gamma = 1.600 +/- 0.008. This value coincides within error estimates with that of gamma = 1.51 +/- 0.04 obtained for a mean perio dogram. Such apparent red noise may be owed to the QPOs. The variation s in UBVRI are highly correlated. A method for determination of relati ve colours of ''slow'', ''fast correlated'' variations and an ''uncorr elated noise'' is proposed. Corresponding parameters are determined. V alues U-B = -1.04(m), -1.24(m), -1.35(m) and B-V = 0.19(m), 0.05(m), 0 .12(m) for ''mean brightness'', ''fast'' and ''slow'' variations, resp ectively.