COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 3299 Konkoly Observatory Budapest 2 March 1989 HU ISSN 0374 - 0676 A MAJOR PERIOD CHANGE IN THE SYSTEM TZ BOOTIS The late spectral type W UMa system TZ Bootis is well known for its unusual photometric behaviour. The extensive investigations of Hoffmann (1) revealed strongly varying lightcurve irregularities attributed partly to underluminous regions on the main component. Further observations (2) showed also the cyclic nature of the variations in the depth of both minima and lead to the assumption of a 3.5 years secondary cycle attributed to a solar like activity cycle. A crucial point for the interpretation of lightcurve changes in close binary systems is the question of the constancy of the period. The interpretation of the presented (O-C) diagram remained ambiguous. Eventually present period changes were obscured by fictitious fluctuations caused by changes in the shape of the lightcurves. These fluctuations showed up in an increased scatter in the (O-C) diagram amounting up to +/-10 min. This was particularly evident in an observational run by Gudur et al. (3). This fact determined us in 1969 to add this star to our regular photoelectric minimum timing program at. Nurnberg Observatory. Since 1984 we were able to secure at least one minimum per year. These minima helped to reveal that the period change suspected by Hoffmann (5) is in fact not spurious and represents the first well documented period shortening observed in that system (fig. 1), so that the ephemeris (I) JD hel. Min I = 24 39632.8418 + 0.2971620d * E. (2) is no longer valid. If we limit our study at the onset of a quasi regular photoelectric survey since the observations of Binnendijk (4) in 1967, the (O-C) diagram (fig.2 ) shows that the observations could be best represented by a set of two elements: from JD 29 40300 to 24 43300 (II) JD hel. Min I = 24 40335.9188 + 0.29716356d * E. +/-9 +/-18 since JD 24 43300 (III) JD hel. Min I = 24 43655.5278 + 0.29715665d * E. +/-13 +/-22 Table 1: Pe. minima from JD 40300 to 43300 with residuals against elements (I) and (II). No. Min.(hel.) Min. Epoch I (O-C)_I (O-C)_II Observer Ref. 1 2439632.8418 I 0 0.0000 +0.0120 Bi 4 2 39636.8506 II 13.5 -0.0029 +0.0091 " " 3 39643.8368 I 37 0.0000 +0.0119 " " 4 40335.7710 II 2365.5 -0.0075 +0.0008 Ca 6 5 40335.9200 I 2366 -0.0071 +0.0012 " " 6 40338.8920 I 2376 -0.0067 +0.0016 " " 7 40345.7260 I 2399 -0.0074 +0.0008 " " 8 40358.8025 I 2443 -0.0061 +0.0021 " " 9 40358.9500 II 2443.5 -0.0071 +0.0010 " " 10 40361.7725 I 2453 -0.0077 +0.0005 " " 11 40361.9200 II 2453.5 -0.0088 -0.0006 " " 12 41356.5284 II 5800.5 -0.0016 +0.0013 Gd/Gl/Hs 7 13 41392.4840 II 5921.5 -0.0026 +0.0002 Rk/Rn " 14 41443.4479 I 6093 -0.0020 +0.0005 Bz/Ib " 15 41450.4280 II 6116.5 -0.0052 -0.0027 " " 16 41453.3970 II 6126.5 -0.0078 -0.0054 " " 17 41462.3130 II 6156.5 -0.0067 -0.0043 Rk/Rn " 18 41465.4360 I 6167 -0.0039 -0.0015 Rk/Hs " 19 41484.4574 I 6231 -0.0008 +0.0014 Ib/Hs " 20 42140.5910 I 8439 -0.0009 -0.0021 Ho 2 21 42151.5888 I 8476 +0.0019 +0.0008 " " 22 42152.4803 I 8479 +0.0019 +0.0007 " " 23 42152.6294 II 8479.5 +0.0029 +0.0012 " " 24 42153.3690 I 8482 -0.0009 -0.0021 " " 25 42153.5203 II 8482.5 +0.0018 +0.0006 " " 26 42156.4917 II 8492.5 +0.0016 +0.0003 " " 27 42470.4463 I 8549 +0.0046 +0.0016 " " 28 42525.4206 I 9734 +0.0039 +0.0007 " " 29 42525.5640 II 9734.5 -0.0013 -0.0045 " " 30 42589.4550 II 9949.5 -0.0001 -0.0037 " " 31 42867.4640 I 10885 +0.0138 +0.0088 " " 32 42869.5375 I 10892 +0.0072 +0.0022 " " Table 2: Pe. minima since JD 43300 with residuals against elements (I) and (III).(: uncertain minimum) No. Min.(hel.) Min. Epoch I (O-C)_I (O-C)III Observer Ref. 33 2443655.5230 I 13537 -0.0008 -0.0048 Ho 2 34 43656.5640 II 13450.5 +0.0001 -0.0038 " " 35 43657.4553 II 13543.5 0.0000 -0.0040 " " 36 43893.5560 I 14338 +0.0054 +0.0058 " " 37 43950.6000 I 14530 -0.0057 -0.0043 " " 38 43974.3775 I 14610 -0.0011 +0.0007 " " 39 43974.5225 II 14610.5 -0.0047 -0.0029 " " 40 44013.4550 II 14741.5 -0.0004 +0.0021 Gb 8 41 44372.4211 II 15949.5 -0.0060 +0.0030 Ho 5 42 44372.5710 I 15950 -0.0047 +0.0043 " " 43 44650.5625 II 16885.5 -0.0083 +0.0058 " " 44 45165.384 : I 18618 -0.020: +0.004 : Gb unpubl. " 45 45814.3651 I 20802 -0.0406 -0.0054 " 9 46 46197.4065 I 22091 -0.0410 +0.0011 Gb/li " 47 46212.4130 II 22141.5 -0.0412 +0.0012 Gb/Be " 48 46610.4523 I 23481 -0.0504 -0.0007 Wu/Be/Li " 49 46909.3909 I 24487 -0.0568 -0.0017 Gb/Li present p. 50 47206.5461 I 25487 -0.0636 -0.0031 GB/Li/Wu " [FIGURE 1] Figure 1. (O-C) diagram, elements I [FIGURE 2] Figure 2. (O-C) diagram, elements III A marked period shortening of 2.3 * 10^-5 P or 0.6 s has taken place in 1977/78. The minima of Binnendijk are not included in the calculations of ephemeris (II), because they lie well above the limits of the usual scatter. We presume that another period change has taken place around the year 1968, but this could not be ascertained with the available observational data. Abbreviations of the observer's names: Be = F. Betten Bi = L. Binnendijk Bz = S. Bozkurt Ca = R.B. Carr Gb = R. Grobel Gd = N. Gudur Gl= O. Gulmen Ho = M. Hoffmann Hs = H. Karacan Ib = C. Ibanoglu Li = G. Lichtschlag Rk = R. Akinci Rn = R. Pekunlu Wu = E. Wunder I would like to thank some members of the Nurnberg astronomical working group for assistance during the measurements. R. GROBEL Nurnberg Observatory Regiomontanusweg 1 8500 Nurnberg 20 F.R.G. References: (1) Hoffmann M. : 1978, Astron. Astrophys. Suppl. 33, 63 [BIBCODE 1978A&AS...33...63H ] (2) Hoffmann M. : 1980, Astron. Astrophys. Suppl. 40, 263 [BIBCODE 1980A&AS...40..263H ] (3) Gudur N. et al.: 1976, IBVS No. 1086 (4) Binnendijk L. : 1969, Astron. J. 74, 211 [BIBCODE 1969AJ.....74..211B ] (5) Hoffmann M. : 1981, IBVS No. 1980, 1980, IBVS No. 1877 (6) Carr R.B. : 1971, Publ. of the Godsell Obs. No. 16 [BIBCODE 1971PGooO..16....1C ] (7) Kizilirmak A. : 1974, IBVS No. 937 (8) Pohl E. et al. : 1978, IBVS No. 1924 (9) Pohl E. et al. : 1985, IBVS No. 2793, 1987, IBVS No. 3087