Astron. Astrophys. 351, 607-618 (1999)

1. Introduction

In 1997 the cataclysmic variable TT Ari showed a new feature. Known as a "negative superhumper" until then, in 1997 TT Ari showed a strong, stable modulation with a period of and a disappearance of modulation (Skillman et al. 1998). The fractional period excess of the new wave is +0.085 and with an orbital period of TT Ari follows the "orbital period-fractional period excess" relation for the CVs showing "positive superhumps" (Stolz & Schoembs 1984). Having this in mind, Skillman et al. (1998) suggested that the modulation could be a "positive superhump" of the same type and that TT Ari is an excellent candidate for investigation of the origin of "superhumps".

Owing to a variety of phenomena observed on time scales from seconds to years, TT Ari is one of the most investigated novalikes. Besides the "3-hour" wave, the photometry of the star on time scales from minutes to days revealed the presence of flickering, 20 min quasi-periodic oscillations (QPOs) and 4 day modulations (Smak & Stepien 1969, 1975; Semeniuk et al. 1987; Udalski 1988; Volpi et al. 1988; Hollander & van Paradijs 1992; Tremko et al. 1996; Kraicheva et al. 1997a; Andronov et al. 1999). In spite of the numerous contributions of these studies to the investigation of the brightness variability of the star, the questions concerning the long-term behaviour of these brightness variations are still open. In the first systematic investigation of the "20-min" oscillations Semeniuk et al. (1987) found that their period decreased from 27 min in 1961 to 17 min in 1985. Kraicheva et al. (1997a) showed that the periods of the QPOs do not decrease continuously. Tremko et al. (1996) and Andronov et al. (1999) found that the peaks in the power spectra (PS) appear around several frequencies predominantly and supposed that contributions of several instability mechanisms with similar time scales are observed. The long-term light curve of TT Ari was discussed by Fuhrman (1981), Hudec et al. (1984), Wenzel et al. (1992), Bianchini (1990) and Kraicheva et al. (1997b). The latter authors found 6 years cyclical brightness variability of the star in high state. The spectroscopic investigations of Cowley et al. (1975) in high state, Thorstensen et al. (1985) in high and intermediate state and Shafter et al. (1985) in low state allowed us to determine the orbital period of TT Ari and to follow its spectral behaviour at different photometric states.

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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