COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1763 Konkoly Observatory Budapest 1980 March 28 RESULTS OF THE FIRST PHOTOMETRIC RUN AT ESO BY THE EUROPEAN WORKGROUP ON Ap STARS In October 1978, several European astronomers decided to cooperate in the frame of an European Working Group on Ap stars, with the aim of improving the coordination of future observation programmes. One of the first initiatives was the application by the Workgroup as an entity for observation time at ESO. The programme resulted from discussions held in Vienna, before the application; and in Paris six months later (by T. Kreidl, Bochum; Gh. Deridder and H. Hensberge, Brussels; F.A. Catalano, Catania; P. Renson, Liege; M. Floquet and M. Gerbaldi, Paris; R. Faraggiana, Trieste; H.M. Maitzen, K.D. Rakosch, M.J. Stift and W.W. Weiss, Vienna). We report here the preliminary results for that part of the programme that concerns the variability of Ap stars. The final paper, including detailed light curves, more accurate periods and a discussion of the results, will be submitted for publication in Astronomy and Astrophysics within the next months. The stars mentioned in Table I were observed with the 50 cm ESO telescope from 1979 November 17/18 to December 6/7 and from 1979 December 22/23 to 1980 January 3/4 with the aim of deriving their rotational periods from the variability in Stromgren u and v filters. In Table II, the main results are summarized. Known and suspected very long period variables (showing in previous runs no variations in time intervals of 10 days) are omitted from this table since observations spread over a much longer time interval are needed to cover the whole cycle. We intend to monitor these slow rotators whenever possible. Other potential observers are kindly encouraged to use the same comparison stars. Table I Summary of observed stars programme stars (Ap) number of filters comparison stars by HD number observations used by HD number 5601 16 u,v 6530,6706 19712 23 u,v 19739,20319 25267 36 u,v 24975,25385 30849 24 u,v 30861*,31587*,31640 38823 20 u,v 38856,38866 50169 2 uvby 50040,50109,50405 52847 2 uvby 52190,52350 53116 12 u,v 53237,53238 55540 2 uvby 55521,55815 56022 23 uvby 55892*,56456 81009 10 u,v 80447,82428 94660 1 uvby 93453,94724 101065 1 uvby 101128,101388,101596 101189 1 uvby 99104,101995,103884 The comparison stars marked by an asterisk are definite variable. HD 30861 got about 0.1m fainter (in u and v) during a 2.5 hour interval in the night 1979 November 19/20, and has therefore immediately been canceled as comparison star. HD 31587 and HD 55892 were observed more frequently (11, resp. 20 observations). These data will be discussed in some detail in the announced paper. The observed range of variation is Delta u~=0.07m, Delta v~=0.05m for HD 31587, and Delta u=0.02m, Delta v~=0.04m, Delta b~< 0.01 and Delta y~=0.02m in the case of HD 55892. The time intervals in which the change from minimum to maximum observed brightness occur are in both cases of the order of 1 - 2 days. Table II Results star name by V magnitude^1, total range approximate HD number peculiarity type Delta u Delta v period (days) 5601 7.64 Si 0.05m 0.04m 1.1? 19712 7.34 Cr Eu .04 .04 2.2 25267 4.64 Si .07 .04 ? 30849 8.86 Sr Eu .05 .10 16 38823 7.33 Sr Eu (F) .07 .14 8.7 53116 8.88 Sr Eu .10 .06 13 ? 56022 4.89 Si .05 .01 0.9^2 81009 6.53 Sr Cr Eu .01 .05 34^3 ^1 V magnitudes are taken from Vogt and Faundez (1979) or Gronbech and Olsen (1976), spectral peculiarity types from Bidelman and McConnell (1973) or Bertaud and Floquet (1974). ^2 see also Renson (1976); Delta b=.015, Delta y=.015. ^3 see also Hensberge et al. (1976) and Wolff (1975). EUROPEAN Ap WORKING GROUP References: Bertaud Ch., Floquet M. 1974, Astron. Astrophys. Suppl. 16, 71 [BIBCODE 1974A&AS...16...71B ] Bidelman W.P., Mac Connell D.J. 1973, Astron.J. 78, 687 [BIBCODE 1973AJ.....78..687B ] Gronbech; B., Olsen E.H. 1976, Astron. Astrophys. Suppl. 25, 213 [BIBCODE 1976A&AS...25..213G ] Hensberge H., de Loore C., Zuiderwijk E.J., Hammerschlag-Hensberge G. 1976, Astron. Astrophys. 48, 383 [BIBCODE 1976A&A....48..383H ] Renson. P. 1976, Astron. Astrophys. Suppl. 25, 143 [BIBCODE 1976A&AS...25..143H ] Vogt N., Faundez M. 1979, Astron. Astrophys. Suppl. 36, 477 [BIBCODE 1979A&AS...36..477V ] Wolff S.C. 1975, Astrophys. J. 202, 127 [BIBCODE 1975ApJ...202..127W ]