COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2522 Konkoly Observatory Budapest 21 May 1984 HU ISSN 0374-0676 ECLIPSING Ap STARS In the Catalogue des periodes observees pour des etoiles Ap, Catalano and Renson (1984) notice that three Ap stars are eclipsing binaries. Although the eclipses of HD 68826 were already studied long ago (see below), HD 34364 is referred to by most authors as the only Ap eclipsing binary (e.g. Hack, 1981, p.90), the results allowing to confirm the nature of the variations of CpD-60d981 and to establish definitely its period have been published only after the achievement of Catalano and Renson's catalogue. Here we call attention to these rare and little known objects, as the study of the changes undergone by their spectra during the eclipses may provide valuable information about the inhomogeneous distribution of the chemical elements over their surface (which would be responsible for the spectroscopic variations observed in most Ap stars, according to the oblique rotator model). The data relevant to these eclipsing binaries are summarised hereafter in Table I. Table I A B C D E F G H I J 34364 17 Aur = AR Aur 5h15.0m +33d43' 6.1 B9 MnHg 4.1347 0.69 0.52 HR1728 - NGC2516-38= - 7h57.4m -60d44' 9.5 A2SrCrEu 3.175 0.17 0.15 CpD-60d981 68826 CoD-48d3586 AO Vel 8h10.4m -48d36' 9.3 B9 Si 1.5846 0.4: 0.2: Column headings: A = HD number, B = other identifications, C = variable star name, D = alpha (1950.0), E = delta (1950.0), F = mv, G = spectral type, H = period (days), I = depth of the primary minimum (mag.), J = depth of the secondary minimum (mag.) The first one, 17 Aur = AR Aur (= HD 34364), has been known for almost a decade as a Mn-Hg star (Wolff and Wolff, 1975). Its eclipses have been studied for more than half a century (see the references quoted by Zverko et al., 1981, and by Catalano and Renson, 1984), while its orbital elements, as a spectroscopic binary, have already been computed by Wyse (1936) and by Harper (1937). Due to the advantageously large inclination of its orbital plane, 88.4d (Johansen, 1970, table 9), a very large fraction of the surface of each component (B8V and B9.5V) is eclipsed in turn. Furthermore, as it is much brighter than the other two stars reported here, spectra may be obtained during its eclipses with a higher signal-to-noise ratio and a better time resolution. Its belonging to the Mn-Hg subgroup of Ap stars may a priori appear. as less favourable, since the spectral variations of these stars, if any, are usually very small (as those of the Am stars, among which more than a dozen eclipsing binaries are known), and the distribution of the various elements over their surface is thus presumably rather homogeneous. However, results of observations obtained in October 1977 during the eclipses by Takeda et al, (1979), partly confirmed by other observations in November 1979 (Takada, 1982), raised several unanswered questions. The second star of Table I, Cox 38 in NGC 2516, is mildly Ap, but belongs to the Sr-Cr-Eu type (Hartoog, 1976), Its eclipsing nature has been known only for a short time (North et al., 1982, North, 1984). The star is rather faint, so that a fast detector is required to get a good insight into the changes of the spectra during the eclipses, which are unfortunately very partial and last for less than four hours. Finally, HD 68826 = AO Vel is a Si star (Bidelman and MacConnell, 1973), i.e. of a type generally displaying rather large variations, so that one can infer that there are significant surface inhomogeneities. While preparing the general catalogue of Ap and Am stars, one of the authors of this note (P.R.) noticed that this star has a variable star name, having long been known to undergo eclipses. Unfortunately, it is almost as faint as the former. That is probably the reason why it has hardly been studied (Hertzsprung, 1937, star II, Oosterhoff and van Houten, 1949) and data on its orbital motion are lacking. During the recent meeting of the European Working Group on Ap Stars (Zurich, March 1984), five members of the group (R. Faraggiana, R. Kroll, G. Mathys, P. North and P. Renson) planned to observe the evolution of the spectra of each of these stars during eclipses. P. RENSON G. MATHYS Institut d'Astrophysique Institut fur Astronomie de l'Universite de Liege ETH-Zentrum 5, avenue de Cointe CH-8092 Zurich (Switzerland) B-4200 Ougree (Belgium) References: Bidelman, W,P. and MacConnell, D.J., 1973, Astron.J. 78, 687 [BIBCODE 1973AJ.....78..687B ] Catalano, F.A. and Renson, P., 1984, Astron.Astrophys.Suppl. 55, 371 [BIBCODE 1984A&AS...55..371C ] Hack, M., 1981, in Upper Main Sequence CP Stars, 23rd Liege Astrophys, Coll. Harper, W.E., 1937, Publ.Dom.Astrophys.Obs., Victoria B.C. 6, 311 [BIBCODE 1937PDAO....6..311H ] Hartoog, M.R., 1976, Astrophys.J. 205, 807 [BIBCODE 1976ApJ...205..807H ] Hertzsprung, E., 1937, Bull.Astron.Inst. Netherlands 8, 157 [BIBCODE 1937BAN.....8..157H ] Johansen, K.T., 1970, Astron.Astrophys. 4, 1 [BIBCODE 1970A&A.....4....1J ] North, P., 1984, Astron.Astrophys.Suppl. 55, 259 [BIBCODE 1984A&AS...55..259N ] North, P., Rufener, F. and Bartholdi, P., 1982, Inf.Bull.Var. Stars No. 2103 Oosterhoff, P.T. and van Houten, C.J., 1949, Bull.Astron.Inst. Netherlands 11, 63 [BIBCODE 1949BAN....11...63O ] Takada, M., 1982, A Peculiar Newsletter 8, 9 (abstract) Takeda, Y., Takada, M. and Kitamura, M.: 1979, Publ.Astron.Soc. Japan 31, 821 [BIBCODE 1979PASJ...31..821T ] Wolff, S.C. and Wolff, R.J., 1975, I.A.U. Coll. No. 32, 503 [BIBCODE 1976paps.coll..503W ] Wyse, A.B., 1936, Publ.Astron.Soc. Pacific 48, 24 and 131 [BIBCODE 1936PASP...48...24W ] [BIBCODE 1936PASP...48..131W ] Zverko, J., Chochol, D., Juza, K. and Ziznovsky, J. 1981, Inf.Bull.Var. Stars No. 1997