COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2525 Konkoly Observatory Budapest 25 May 1984 HU ISSN 0374-0676 * UBVRI PHOTOMETRY OF SOUTHERN ACTIVE CHROMOSPHERE STARS During the past few years the active chromosphere stars received much attention because of the variety of extraordinary phenomena exhibited by them. Since the majority of these interesting objects lies north of -40d declination, the discovery of the new bright southern active stars is an important task. Based on medium dispersion spectra, the selection of the potential southern active chromosphere candidates was done by several authors: Bidelman and MacConnell (1973), Weiler and Stencel (1979), Hearnshaw (1979), Bopp and Hearnshaw (1983). In all these papers the most important criterion for chromospheric activity was, the abnormally strong emission in absorption lines of Ca II H and K and/or H alpha. As the majority of the known chromospheric active stars shows intrinsic small amplitude light variations (up to 0.3 mag) we have undertaken a photometric program for searching for new active stars. Based on the above mentioned candidate lists we have chosen six stars for Which, as we know, photometry has not yet been done (except for two observations of HD 155555 - Eggen, 1978). We have selected stars showing extremely strong H and K Ca II and/or H alpha - emission, which suggest good chances for finding optical variability (in analogy to the well known northern RS CVn systems like HR 1099 or UX Ari). In this paper we present very preliminary results of UBVRI photometry of chosen stars. The observations were carried out at the European Southern Observatory at La Silla, during 12 consecutive nights from 13 April 1984 on. The ESO 50 cm telescope equipped with single beam photometer housing the RCA 31034 gallium-arsenide photomultiplier in conjunction with a standard UBVRI set of filters as described by Bessel (1979) was used. The observations were made differentially absolute photometry of the comparison stars was obtained during 9 nights under the best weather conditions. During these nights at least 30 standards were observed to tie in the comparison stars to the * Based on observations collected at the European Southern Observatory, La Silla, Chile standard UBVRI system, The constancy of the main comparison was checked several times per night during the first few nights and at least two times later, when it was realized that they are stable. The standard errors for a single observation are 0.015m, 0.008m, 0.007m, 0.008m, 0.005m for the UBVRI colour bands, respectively. Table I Summary of the UBVRI photometry of the southern active chromosphere stars Object Sp Main Secondary Variability V Remarks Comparison Comparison Ampl. (mag) HD 86005 K2IIIp HD 86034 HD 85849 NO - 1 HD 102077 K0/1Vp HD 102076 HD 102202 FOUND 0.08 HD 119285 K1Vp HD 119164 HD 119076 ? <0.03 2 HD 127535 K1IV/Ve HD 128227 HD 128618 FOUND 0.24 HD 139084 K0V HD 139070 HD 139002 FOUND 0.11 HD 155555 K1Vp HD 156427 HD 154775 FOUND 0.08 * According to Houk and Cowley (1975) and Houk (1978). Remarks: 1. Mean colours: V=7.230m B-V=1.307m U-B=1.026m V-R=0.711m V-I=1.347m m.e. .005 .007 .011 .003 .005 2. Variability possible, but with the period longer than duration of our run (12d). Mean colours: V=7.856m B-V=1.092m U-B=0.831m V-R=0.636m V-I=1.241m m.e. .017 .007 .015 .005 .010 Evidence for photometric variability was found for 4 stars from our sample. In Table I we list the summary of our observations, i.e. HD number of objects and comparisons. For the found variability we also give the V amplitudes and for the remaining stars all colours. The more detailed analysis and light curves will be published in forthcoming papers. A. UDALSKI* and E.H. GEYER Observatorium Hoher List der Universitats-Sternwarte Bonn D-5568 Daun, F.R.Germany * On leave from the Warsaw University Observatory, Poland References: Bessel, M.S., 1979, Pub.A.S.P. 91, 589 [BIBCODE 1979PASP...91..589B ] Bidelman, W.P. and MacConnell, D.J., 1973, A.J., 78, 678 [BIBCODE 1973AJ.....78..687B ] Bopp, B.W, and Hearnshaw, J.B., 1983, Ap.J. 267, 653 [BIBCODE 1983ApJ...267..653B ] Eggen, O.J., 1978, Inf.Bull.Var. Stars No. 1426 Hearnshaw, J.B., 1979, in I.A.U. Colloquium 46, p.371 [BIBCODE 1979ctvs.conf..371H ] Houk, N., 1978, Michigan Catalogue of Two-Dimensional Spectral Types for the HD Stars, Vol. 2 [BIBCODE 1978mcts.book.....H ] Houk, N. and Cowley, A., 1975, Michigan catalogue of Two-Dimensional Spectral Types for the HD Stars, Vol. 1 [BIBCODE 1975mcts.book.....H ] Weiler, E.J. and Stencel, R.E., 1979, A.J. 84, 1372 [BIBCODE 1979AJ.....84.1372W ]