COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 992 Konkoly Observatory Budapest 1975 April 25 OBSERVATIONS OF 26, 27 AND 28 CMa In an attempt to learn a bit more about their variability 27 and 28 CMa were observed by the author on 11 nights in January 1970 with the 6" Zeiss telescope on La Silla Mountain. 26 CMa was used as a comparison star together with HD 55857. But both comparison stars proved to be (variables themselves. One of them, HD 55857 turned out - as already reported in IBVS No 807 - to be a beta CMa with a period of only 2h40m and an amplitude <=.02m in V. Corrected for the corresponding small fluctuations, the deflection curves for that star differed so very little among themselves that it seemed safe to conclude from them that the average brightness of HD 55857 did not suffer night to night variations and that it could be used as a standard for the evaluation of the three other stars' brightnesses and their variations. This was actually done and the magnitude 6m.11 adopted for m_{HD 55857} in accordance with the value given by Cousins and Stoy (1). The comparison with the deflection curves of the three other stars gave the following results. 1. 26 CMa (=HD 55522, alpha_{1900}=7h08m07s, delta_{1900}=-25deg47', m=5.86, S=B 3)^* No significant variation could be seen in the course of the 5h - 6h runs that were made on the star, but there were definite changes from one night to another in the difference Delta(m(HD 55522-average(HD 55857))) as illustrated by Figure 1. The dots representing these differences lie satisfactorily on a sine curve with period P = 2d.68 and amplitude Delta(m)=0m.037. Since no star in the spectral range B 0 - B 3 is known to pulsate with such a long period, it seems plausible to admit that we are dealing with a spectroscopic binary, in which case the photometric period just given has to be doubled to yield the orbital period P_{orb}=5d.36. Spectroscopic observations are needed to check this point. ^* The data within brackets are taken from the "Catalogue of Bright Stars" 3d edition, 1964. 2. 27 CMa (=HD 56014, alpha_{1900}=7h10m11s, delta_{1900}=-26deg11', m=4,42, S= B 3 III pe) The star is classified as a unique variable under the name EW CMa in volume I of the 3d edition of the G.C.V.S. and an extensive bibliographical note is devoted to it on p.417^{**} of that volume. The first aim of our photometric investigation was to find out whether the short period 0d.261975 discovered in the RV variations by Ringuelet-Kaswalder (2) and which could be traced back - though with a considerably reduced amplitude - in our own RV measurements of December 1969 and January 1970 (see Table 1 and Figure 2) would also be detectable in the brightness variation. The result of this investigation was negative; the shots made on the star in one and the same night showed nothing but an irregular scatter of a few thousands of a magnitude around a mean that stayed stable, at 1m.46 below HD 55857, over the whole observation campaign. Putting as already mentioned average(m_{HD 55857})=6m.11 we thus find m_{27 CMa}=4m.65. This value is exactly the one which the G.C.V.S. mentions for the minimum observed brightness of the star, its maximum being 4m.3. Table 1 RV's of 27 CMa Date JD RV Date JD RV -2440000 km/sec phase -2440000 km/sec phase 1969 1969 Dec 21 576d.7102 - 32.0 .711 Dec 28 583d.8416 - 33.9 .933 22 77.8457 - 35.0 .045 29 84.8298 - 36.6 .705 23 78.7930 - 37.5 .611 30 85.8513 - 31.0 .604 24 79.7500 - 29.0 .314 1970 25 80.8347 - 27.1 .455 Jan 21 607.5356 - 25.0 .665 26 81.8359 - 34.0 .277 .7209 - 15.5 .372 27 82.8416 - 33.2 .115 .8682 - 12.4: .118 22 08.5503 - 22.1 .538 .7175 - 16.5 .177 Phases are for period 0d.261975 and origin JD 2440576d.0000 3. 28 CMa (=omega CMa=HD 56139,alpha_{1900}=7h10m45s,delta_{1900}=-26deg36', m=3.83, S=B 3 IVe) The star is classified in the G.C.V.S. (1969) as of type Ia, with observed brightness limits 3m.82 and 4m.04 in V. Besides the eleven nights on La Silla in 1970, fractions of five more nights were devoted to it in March 1973 at Boyden, where the star was observed photometrically with the 60" telescope. ^{**} It is somewhat misleading that the reference numbers <=[0596] there given do not correspond to those of the "Bibliography" beginning p. A. 42 of that volume, but do refer to those of the "Literature" beginning p. 365 of the 2d Supplement to the 2d edition (1967) of the G.C.V.S. Its magnitude, averaged over the night, and derived from its difference with the average magnitude 6m.11 of HD 55857 varied as Table 2 shows Table 2 Night-averages of m_{28 CMa} Date m Date m 1970 Jan 4/5 3.62 1970 Jan 16/17 3.64 5/6 3.62 22/23 3.64 6/7 3.63 24/25 3.66 9/10 3.63 1973 Mar 15/16 4.15 10/11 3.60 20/21 4.17 12/13 3.63 21/22 4.17 13/14 3.63 22/23 4.18 14/15 3.65 30/31 4.18 So, the limits of the brightness variation given in the G.C.V.S. are to be widered seriously. But not only the night average changed slightly from one night to the next; there were also slight changes visible in the course of the 6h runs on some nights. They suggested variations with a characteristic interval of 20h-22h, but efforts to find a real period of that order remained unsuccessful. It is perhaps not out of order to reproduce here the results of RV observations made with the 152 cm telescope on two other nights in January 1970 on La Silla - and reported already elsewhere (3) - since they too indicate a rather quick variation. Table 3 RV's of 28 CMa Date U.T. JD RV_{absor} RV_{em} km/sec km/sec 1970 Jan 21 0h52m 2 440607d.5398 +34 +21.5 5 32 .7341 +41 - 9.7 8 51 .8730 +39 +17.1 22 1 15 08.5557 +12 +26.8 5 13 .7217 +11 +28.0 5 58 .7522 +21 +23.1 A. VAN HOOF Astronomisch Instituut Katholieke Universiteit Leuven Naamsestraat 61 B-3000 Leuven, Belgium References: (1) A.W.J. Cousins and R.H. Stoy, Royal Obs.Bull. No 64, 1963 [BIBCODE 1963pmcs.book.....C ] (2) A.E. Ringuelet-Kaswalder, Ap.J. 135, 755, 1962 [BIBCODE 1962ApJ...135..755R ] (3) A. Van Hoof, Mededelingen v.d.Kon.Acad.v.Wet.Lett. en Sch.K. v. Belgie, XXXV, No 4, 1973 [FIGURE 1] [FIGURE 2]