COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 990 Konkoly Observatory Budapest 1975 April 16 ON THE PERIOD VARIATION AND BLAZKO EFFECT OF XZ CYGNI Between April 16, 1972 - October 17, 1974, 2870 B.V. photoelectric observations were made on XZ Cygni with the 50 cm reflector at the Astronomical Observatory of the University of Cluj-Napoca (Romania). Table 1 shows the moments of 38 maxima and the corresponding O-C differences calculated with the elements Max.hel. = J.D. 2441453.3856+0d.4664731.E (1) Using the maxima from Table 1, as well as those published by Kunchev (1974), we listed the normal maxima in Table 2 where the O-C differences were calculated with the linear elements from GCVS (1969). Using the normal maxima published by Lange and Gusev (1972) and those from Table 2, we drew up Fig.1 which shows the variation of the O-C values versus the number of main cycle E. It is obvious that, after 1965, a real and very rapid decrease of the fundamental pulsation period of XZ Cygni has occurred.Assuming that the period decrease is proportional to the time, there results a decrease rate of the order of 1.32 x 10^-5 days per year. This decrease can be explained on the basis of the star evolution during the instability strip crossing (Kukarkin, 1974), or may be caused by the mass loss which might be significant in the RR Lyrae phase (Laskarides, 1974). Wesselink (1974) shows that theoretical calculations predict the existence of the variables evolving towards the red edge of the instability strip if their period is increasing. This does not necessitate that XZ Cygni, whose period decreases, evolves towards the blue edge of the instability strip. Figs.1 and 2 show the variation of the O-C differences versus E for the years 1971-1972 and 1973-1974, respectively. The 1971 maxima were visually observed by Bogdanov (1972). The upper part of Fig. 3 represents the height variation of the maximum versus E. All the O-C values shown in Figs.2 and 3 where calculated by means of the elements (1). [FIGURE 1] [FIGURE 2] [FIGURE 3] Table 1 Max.hel. O-C Max.hel. O-C Max.hel. O-C JD 2400000+ JD 2400000+ JD 2400000+ 41424.4425 -0.0218 41856.4236 +0.0052 41982.3762 +0.0101 453.3970 +0.0114 862.4865 +0.0040 988.4330 +0.0028 461.3390 +0.0234 869.4750 -0.0046 997.2900 -0.0032 467.3858 +0.0060 910.5330 +0.0038 42047.2115 +0.0056 468.3187 +0.0060 913.3310 +0.0029 161.5030 +0.0112 507.5088 +0.0124 932.4585 +0.0050 162.4362 +0.0115 515.4295 +0.0030 933.3933 +0.0069 245.4580 +0.0011 522.4295 +0.0059 939.4507 +0.0001 280.4533 +0.0109 544.3180 -0.0298 947.3688 -0.0118 288.3865 +0.0141 622.2557 +0.0068 960.4355 -0.0064 308.4492 +0.0184 834.4698 -0.0243 962.3077 +0.0000 331.3068 +0.0188 835.3993 -0.0278 967.4460 +0.0070 338.3025 +0.0174 849.4265 +0.0052 974.4430 +0.0070 Table 2 Max.hel. O-C E No. Max.hel. O-C E No. JD 2400000+ max. JD 2400000+ max. 41461.3221 -0.3415 9704 5 41932.4553 -0.4531 10715 6 522.4183 -0.3672 9835 5 962.3088 -0.4607 10778 4 544.3247 -0.3900 9882 5 988.4332 -0.4647 10834 3 565.3515 -0.3592 9927 5 42047.2115 -0.4753 10960 1 573.2650 -0.3776 9944 5 162.4362 -0.4956 11207 2 599.3781 -0.3929 10000 5 280.4531 -0.5232 11460 3 849.4138 -0.4435 10537 6 333.3074 -0.5260 11569 3 The analysis of these diagrams shows that: (a) There is not an exact reproduction of the secondary cycle. An average period Pi = 58d.15 can be inferred. (b) An increase of the secondary period Pi corresponds to a decrease of the fundamental period P like in the case of RW Draconis. (c) The Delta(V_max) curve is shifted by 0.19 of the secondary cycle as compared with the O-C curve. (d) The amplitude of the O-C diagram decreased from 1971 till 1974. This phenomenon might go on, like in the case of RW Draconis and RR Lyrae, up to the minimum (or "disappearance") of the Blazko effect and, after that, there might occur a new amplitude increase of this effect. The explanation of the "disappearance" in 1963 of the Blazko effect of the RR Lyrae was given by Zaikova et al. (1973). For the study of the evolution of these effects and more detailed investigations of XZ Cygni pulsations, simultaneous photoelectric and spectrophotometric observations are necessary. VASILE POP Astronomical Observatory of Cluj-Napoca References: Bogdanov, M.B. 1972, The Variable Stars, Suppl. 1, 5, 309 (Russ) [BIBCODE 1972PZP.....1..309B ] Kukarkin, B.V., et al. 1969 General Catalogue of Var.Stars., Moscow [BIBCODE 1969gcvs.book.....K ] Kukarkin, B.V. 1974, RR Lyrae and W Virginis Type Stars., Preprint, Symposium IAU No. 67., Moscow Kunchev, P. 1974, I.B.V.S. No. 927 Lange, G.A., Gusev, P.P. 1972, The Variable Stars.,Suppl. 1, 5, 333 (Russ) [BIBCODE 1972PZP.....1..333L ] Laskarides, P.G. 1974, Astrophys.Space Sci. 27, 2, 485 [BIBCODE 1974Ap&SS..27..485L ] Wesselink, A.J. 1974, Astron.Astrophys. 36, 163 [BIBCODE 1974A&A....36..163W ] Zaikova, L.P., Lysova, L.E., Romanov, Yu.S., Tsessevich, V.P., Shakoon, L.J. 1973, Astron. Circ. 787, 4, (Russ) [BIBCODE 1973ATsir.787....4Z ]