COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS NUMBER 542 Konkoly Observatory Budapest 1971 April 23 PHOTOELECTRIC OBSERVATIONS AND TIMES OF MINIMA OF Y Cygni UBV observations of the eclipsing binary Y Cygni were made by the writer, between 1967 and 1969, with a 1P21 photomultiplier (cooled) on the 60 cm Zeiss reflector of the Vatican Observatory. The comparison star used was BD +34d4196 (B9), which is conveniently near to the variable in position and colour. The V magnitude and B-V, U-B colours of 34d4196 were determined from comparisons with Johnson- Morgan standard stars on several nights. They are given in Table 1, together with their probable errors. In the table are also given the V magnitude and colours of Y Cygni outside eclipse. The observations outside eclipse were not sufficiently spaced to determine the shape of the light curve, and the values given are simply the means of all the observations made outside eclipse. The probable errors of these values, when compared with those of the comparison star, go to indicate that the light-curve outside eclipse is not flat. Table 1 ----------------------------------------------------------------- V B-V U-B Number of obs. nights ----------------------------------------------------------------- BD +34d4196 7.948m -.065m -.303m 22 9 +-.003 +-.003 +-.005 (p.e.) Y Cyg- 4196 -0.651 -.007 -.660 19 8 (outside eclipse) +-.004 +-.006 +-.015 (p.e.) Y Cyg- 4196 -0.050 -.006 (mid-Minimum I) ---------------------------------------------------------------- The V magnitude and B-V of Y Cygni at the middle of Minimum I are also given in the table, from the only minimum observed, that of the night of November 2/3, 1969. During that minimum, I made 83 observations of Y Cygni in B and V, each observation being the mean of two measures. The time of minimum was determined as; Minimum I = JD hel. 2440528.2577 +-.0005 (p.e.) This is epoch no. 10344 from the elements given by Dugan (Princeton Cont. No. 12, p. 28, 1931; Dugan's formulae are also given by H. Schneller in Information Bulletin No. 61, 1964). The O-C of this minimum from that computed from Dugan's formula, using both sinusoidal terms, is -0.0047d. The smallness of this correction, about seven minutes, goes to show both the regularity of the apsidal motion and the accuracy of Dugan's elements. The period of Y Cygni is so unfavourable that it is impossible to observe the entire light curve from one observatory within a reasonable interval of time. Accordingly it was decided by IAU Commission 42, during the Brighton meeting, to organize an international campaign, with observers spread over a wide range of longitude. Professor Mario G. Fracastoro, (Director, Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy) and myself are the coordinators of the campaign. Table 2 Predicted Minima of Y Cygni, June-November 1971 ------------------------------------------------------------------------ Minimum I Minimum II Minimum I Minimum II Epoch JD hel. JD hel. JD hel. JD hel. 2441 2441 2441 2441 ------------------------------------------------------------------------ 10540 115.553 117.294 10570 205.446 207.182 541 118.550 120.290 571 208.442 210.178 542 121.546 123.286 572 211.439 213.174 543 124.542 129.283 573 214.435 216.170 544 127.539 129.279 574 217.431 219.167 545 130.535 132.275 575 220.428 222.163 550 145.517 147.256 580 235.410 237.144 551 148.514 150.253 581 238.406 240.140 552 151.510 153.249 582 241.403 243.137 553 154.507 156.245 583 244.399 246.133 554 157.503 159.241 584 247.396 249.129 555 160.499 162.238 585 250.392 252.125 560 175.482 177.219 590 265.374 267.107 561 178.478 180.215 591 268.371 270.103 562 181.474 183.211 592 271.367 273.099 563 184.471 189.208 593 274.363 276.095 564 187.467 189.204 594 277.360 279.092 565 190.464 192.200 595 280.356 282.088 ------------------------------------------------------------------------ In Table 2 are listed the predicted times of both minima from June to November 1971. The times were computed with Dugan's elements, using both sinusoidal terms. A correction of -.0047d (found from my observed minimum) was applied to the computed times of Minimum I. No correction was applied to the computed times of Minimum II; they should not be in error by more than a few minutes, at most. The epoch numbers given in the table are those or Dugan's elements. Minima around the time of Full Moon were omitted from the table. During this interval the mean period of Minimum I may be taken as 2.996418d; and that of Minimum II as 2.996258d. 12 April, 1971 D.J.K. O'CONNELL Borgo S. Spirito 5 00193 Roma, Italy