COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2269 Konkoly Observatory Budapest 1983 January 19 HU ISSN 0374-0676 uvby OBSERVATIONS OF V471 Tau IN OCTOBER 1981* The red dwarf - white dwarf binary V471 Tau is for many reasons one of the most interesting close binaries. One of the aspects is the spot activity of the K2V component due to the fast, orbitally synchronized rotation (P=0.521 d). The time scale of spot re-arrangements on this star is very short, of the order of days, as documented in a number of studies, e.g. in the 1976 uvby photometry on 9 consecutive nights by Rucinski (1981). Here, new uvby observations obtained also on 9 consecutive nights of the last decade of October 1981 are presented. They have been collected with the simultaneous four-colour photometer on the 50 cm Danish telescope in ESO, La Silla, Chile (Gronbech et al. 1976). Up to 40 standard stars were observed each night and the following data for the comparison star HD 24040 have been obtained from 9 independent tie-ins to the standard system: V = 7.508 +/- 0.002, (b-y) = 0.412 +/- 0.001, m1 = 0.221 +/- 0.001, c1 = 0.380 +/- 0.002. These values differ slightly from those given in Rucinski (1981) and are probably of a higher quality than the previous data. The differential observations of V471 Tau are listed in Table I and are plotted in Fig. 1 together with the 1976 light curve. The observations were obtained normally only once per night, at culminations through air-masses about 1.47. The time in Table I is expressed in heliocentric modified Julian Days (MJD.= JD - 2 400 000.5); the phases have been computed from the ephemeris of Oliver and Rucinski (1978) for the segment C of their (O-C) diagram: MJD 40609.56642 + 0.52118294 E. ** Based on observations obtained at the European Southern Observatory, La Silla, Chile. Table I Differential observations of V471 Tau (relative HD 24040) MJD (hel) Phase DeltaV Delta(b-y) Deltam1 Deltac1 44898.235 0.721 1.924 0.116 0.099 -0.271 .300 .844 1.941 0.108 0.121 -0.292 44899.278 .721 1.929 0.097 0.125 -0.240 44900.248 .582 1.965 0.117 0.093 -0.289 44901.250 .506 1.966 0.118 0.101 -0.315 44905.203 .088 1.961 0.102 0.113 -0.254 .282 .241 1.941 0.113 0.105 -0.294 44906.231 .063 1.953 0.108 0.117 -0.271 44907.254 .024 1.973 0.118 0.120 -0.164 44908.255 .946 1.946 0.112 0.097 -0.248 44909.253 0.861 1.944 0.112 0.108 -0.257 [FIGURE 1] Fig.1 The October 1981 observations of V471 Tau plotted on the phase diagram together with the 1976 lighy curve. The new observations indicate that the variability of V471 Tau almost disappeared in October 1981 with the brightness level (DeltaV = 1.95) stabilized close to the maximum brightness observed in 1976 at phase 0.81 (DeltaV = 1.92). The mean differential indices (all observations outside eclipse): Delta(b-y) = 0.110 +/- 0.002, Deltam1 = 0.108 +/- 0.003, Deltac1 = -0.273 +/- 0.008 were also close to those observed at the light maximum in 1976: Delta(b-y) = 0.108, Deltam1 = 0.110, Deltac1 = -0.288 but differed from those at the light minimum in 1976 at phase 0.5: Delta(b-y) = 0.135, Deltam1 = 0.095, Deltac1 = -0.360. This sheds new light on the unexplained peculiarity of colours at light minimum noticed previously and suggests that this peculiarity was not caused by the reflection effect of the white dwarf light. These observations have been collected at ESO, La Silla during my association with the Max-Planck-Institut fur Astrophysik, Munich; I am grateful to the Directors of the Institute for extending its excellent facilities to me. S. M. RUCINSKI Institute of Astronomy Madingley Road Cambridge CB3 OHA, England (on leave from the Warsaw University Observatory) References: Gronbech, B., Olsen, E.H., Stromgren, B.: 1976, Astron.Astrophys.Suppl., 26, 155. [BIBCODE 1976A&AS...26..155G ] Oliver, J.P, Rucinski, S.M,: 1978, IBVS, No. 1444. Rucinski, S.M.: 1981, Acta Astr., 31, 37. [BIBCODE 1981AcA....31...37R ]