4 Comparison with other periodicities

The flare at 7 mm is clearly in contrast with the above given scenario of inter-binary collision at periastron. In fact, it occurs almost at the apoastron. Even if one postulates a fundamentally different nature for the millimeter emission (Skinner et al. 1997) the occurrence of another independent process exactly in the middle of the 52 day cycle is rather unlikely. The periodicity of 52 days might only accidentally be coincident with the orbital period. If the data are folded with 52 days (Fig. 6)

Figure 6: Radio observations of V773 Tau folded with the period of 52 days found in the spectral analysis.

there is an alignment at phase 0.5 of the flare at 7 mm with one at 2.8 cm. The time between Phase 0 and 0.5 corresponds to a time interval of 26 days.

Looking more deeply into the solar literature we have discovered that Bai (1987) found a periodicity of 51 days in the occurrence rate of flares during the solar cycle 19. Delache & co-authors (1985) found periods at 51 days and 153 days analyzing the Zürich daily sunspot number of a 10 years interval. From solar neutrino flux data again periods of 52 and 157 days have been found (Sturrock et al. 1999). This long period between 152 and 160 days which sometimes appears in the Sun was first discovered by Rieger et al. in $\gamma$-ray and X-ray flares (Rieger et al. 1984) and afterwards confirmed also for H$\alpha$ flares, flares at radio wavelengths and even for variations of the solar diameter (see references in Oliver & Ballester 1995).

Bai & Sturrock (1993) rose the question of a comparison with helioseismological data and suggested the solar periods around 51 and 153 days to be just sub-harmonics of a fundamental period that they found to be 25.5 days. Indeed, analyzing the power spectrum of sunspot area data over more than 110 years Bai (1999) confirms the 25.5 day period as the most persistent period in the Sun.

The similarity with UX Arietis and its periods of 25.5, 56 and 158.7 days discussed in the introduction is very impressive. In conclusion: It appears that there exists an intrinsic mechanism causing the periodic emersion of loops. The periodicity appears to be a multiple of 25.5 days in the Sun. Similar periodicities have been found in the evolved star UX Arietis. Does this picture apply as well for V773 Tau? If that would be the case, this periodicity would be independent of age and rotational period but dependent from the stellar mass, quite similar for all these three stellar systems.