COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 4042 Konkoly Observatory Budapest 30 June 1994 HU ISSN 0374 - 0676 ON THE STABILITY OF SPOTTED REGIONS ON STELLAR SURFACES OF WEAK-LINE T TAURI STARS We undertake a long-term photometric monitoring of a sample of WTTS in the Tau- Aur complex aimed at detecting periodic processes in the light curves of above stars and determining the lifetimes of cool spots on the star's surfaces (Grankin 1992, 1993). Maidanak UBVR-photometry Data Bank is at Tashkent Astronomical Institute and available to any interested in. Table 1 HBC Name Period (day) Observ. # bo93 gr93 other source This interval paper 1900+ 29 V410 Tau 1.871(vr88) 1.872095 86-93 68 VY Tau 5.37(gr91) 5.36995 85-93 370 LkCa 4 3.37 3.37 3.36(vr93) 3.3745 85-93* 376 TAP 26 2.51 2.52 2.50836 92-93 378 V819 Tau 5.6(ry84) 5.5354 82-93* 379 LkCa 7, TAP 29 5.64 5.66(gr92, vr93) 5.6638 85-93* 380 HDE 283572, TAP 31 1.55(wa87) 1.529 92-93 388 TAP 35 2.74 2.73 92-93 TAP 39 3.67 3.654 92-93 392 TAP 40 1.55 1.555 92-93 397 TAP 41 2.43 2.426 92-93 399 V827 Tau 3.75(bo86) 3.75886 81-93* 403 TAP 45 6.2 9.91 92-93 405 V830 Tau 2.76(ry84) 2.74079 82-93* 407 TAP 49 3.32 3.32 92-93 408 Wa Tau/1, TAP 50 3.06 93 + 420 LkCa 16 5.6 5.6 92-93 426 LkCa 19, TAP 56 2.24 2.24 2.236 90-93 427 TAP 57NW 9.32(za93) 9.345 92-93 429 V836 Tau 6.99(ry84) 6.755 82-93* References to Table 1: bo86: Bouvier et al. (1986); bo93: Bouvier et al. (1993); gr91: Grankin et al. (1991); gr92: Grankin (1992); gr93: Grankin (1993); ry84: Rydgren et al. (1984); vr88: Vrba et al. (1988); vr93: Vrba et al. (1993); wa87: Walter et al. (1987); za93: Zakirov et al. (1993) # - according to Herbig and Bell (1988); * - it was used both our observations and others; + - this star was included in our observation program in 1993 [FIGURE 1] Figure 1. Phase diagrams for light curves in the V filter for 5 WTTS. Empty circles - data of 1992, filled circles - data of 1993. [FIGURE 2] Figure 2. Phase diagrams for different seasons of TAP 29 (LkCa 7). For the 1985/86 season data from Vrba et al. (1993) are also shown. During observations from 1992 to 1993 we did not find any essential changes in the phase and frequency of the periodic processes for most of the WTTS with well-known periods (see Table 1 and Figure 1). Moreover, three stars (LkCa4, LkCa7, and VY Tau) keep their periods in 1985-1993 and four ones (V819 Tau, V827 Tau, V830 Tau, and V836 Tau) in 1982-1993. Only some small changes were found in the shape and the amplitude of their folded light curves (e.g. Figure 2). It is known that the periodic variability of WTTS is mostly due to dark spots on their surfaces. The strict periodicities in the light curves of observed stars allow to suggest that the lifetimes and localization of the spot groups on the stellar surfaces do not change on the timescale from one to a few years. Hitherto only one WTTS (V410 Tau) with similar properties is known (Vrba et al., 1988). The phenomenon of the spotted region stability on stellar surfaces seems to be explained either by strong local magnetic fields or the tidal interaction in the close binary systems (as RS CVn type). K. N. GRANKIN Astronomical Institute Astronomical str. 33, Tashkent 700052 Uzbekistan, CIS References: Bouvier, J. et al., 1986, Astron. and Astrophys., 165, 110 [BIBCODE 1986A&A...165..110B ] Bouvier, J. et al., 1993, Astron. and Astrophys., 272, 176 [BIBCODE 1993A&A...272..176B ] Grankin, K. N. et al., 1991, IBVS, No. 3658 Grankin, K. N., 1992, IBVS, No. 3720 Grankin, K. N., 1993, IBVS, No. 3823 Herbig, G. H. and Bell, K. R., 1988, Lick. Obs. Bull, No. 1111 [BIBCODE 1988cels.book.....H ] Rydgren, A. E. et al., 1984, Astron. J., 89, 1015 [BIBCODE 1984AJ.....89.1015R ] Vrba, F. J., Herbst, W., Booth, J. F., 1988, Astron. J., 96, 1032 [BIBCODE 1988AJ.....96.1032V ] Vrba, F. J. et al., 1993, Astron. J., 106, 1608 [BIBCODE 1993AJ....106.1608V ] Walter, F. M. et al., 1987, Publ. Astron. Soc. Pac., 99, 31 [BIBCODE 1987PASP...99...31W ] Zakirov, M. M., Azimov, A. and Grankin, K. N., 1993, IBVS, No. 3898