COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS NUMBER 864 Konkoly Observatory Budapest 1974 January 25 AN ORBITAL PERIOD FOR VW HYDRI The dwarf nova VW Hyi has been observed since 1968 at the Auckland Observatory using the 50 cm telescope and associated photoelectric equipment. Although this system does not exhibit eclipses, in earlier papers in "Southern Stars" we drew attention to repetitive features in the light curve which indicated a period of around 100 minutes. More extensive observations now indicate a period of 106.95 minutes. We have extended our series of observations to provide a base line of over 25,000 cycles. Using all these which show unambiguous repetitive features, as listed in Table I, we have derived the elements given below for a bright regular feature of the light curve. Our curves have not been treated to remove the Table I Observed Times of Peak on 'Minimum Light' Light-Curves HJD Cycle No. O - C HJD Cycle No. O - C 2440000+ 2440000+ 128.0222 0 1620.8785 20100 +0.0072 128.0953 1 -0.0012 1620.9549 20101 +0.0093 394.1375 3583 +0.0021 1633.9444 20276 +0.0014 394.2153 3584 +0.0055 1634.0177 20277 +0.0004 456.0826 4417 +0.0050 1778.9264 22228 +0.0062 456.1581 4418 +0.0062 2005.8924 25284 -0.0003 470.1125 4606 -0.0024 2005.9661 25285 -0.0009 484.0049 4793 +0.0013 2006.0404 25286 -0.0008 484.1549 4795 +0.0028 2006.9316 25298 -0.0009 well known rapid irregular flickering which is present in VW Hyi, as in other cataclysmic variables. Elements: H.J.D. 2,440,128.0222 + 0.0742711d E +- 0.0000002 These elements provide a reliable ephemeris for the system at minimum light, subject to the expected uncertainties of a feature of this type. Figures 1 and 2 show typical light curves. The points shown are from 30 second integrations in white light using a blue-sensitive EMI 9502 photomultiplier tube. [FIGURE 1] [FIGURE 2] Once the star brightens more than two magnitudes (as during outbursts) this feature quickly disappears. It may be replaced by other similar appearing peaks but these are usually displaced in phase and cannot be fitted with any degree of certainty to the foregoing elements. Table II lists some features observed during the recent supermaximum outburst. These show a change Table II Observed Times of Peak on 'Minimum Light' Light-Curves HJD Cycle No. O - C Phase HJD Cycle No. O - C Phase 2442000+ 2442000 010.9632 25352 +0.0201 0.27 020.0198 25474 +0.0156 0.21 011.0368 25353 +0.0194 0.26 020.9383 25486 +0.0428 0.58 011.1097 25354 +0.0180 0.24 020.9638 25487 +0.0059 0.08 012.8924 25378 +0.0182 0.25 021.0157 25482 +0.0460 0.62 012.9660 25379 +0.0176 0.24 021.9322 25500 +0.0031 0.04 019.8885 25472 +0.0328 0.44 021.9611 25500 +0.0258 0.35 019.9448 25473 +0.0149 0.20 022.0047 25501 -0.0048 0.94 019.9684 25473 +0.0385 0.52 022.0385 25501 +0.0290 0.39 in phase, and in some cases an occurrence or more than one peak per cycle. Generally the quiescent peaks are 0.5 to 0.9 magnitudes amplitude. Those noted in Table II are pretty much the same but tended to reduce in amplitude in the later stages At times the system exhibits flare-like behaviour which can completely submerge the more normal light peak. This is especially pronounced at times between phase 0.60 and 0.90. A small 'flare' of this type is shown in Figure 2. Confusion over these, and attempts to fit the peaks occurring during outbursts, can lead to a variety of incorrect orbital periods for this star, and led to our earlier, too short, period. BRIAN F. MARINO W. S. G. WALKER Royal Astronomical Society of New Zealand 18 Pooles Road Greerton, Tauranga, New Zealand. References: Southern Stars Vol. 23 No. 6 p 124 Dwarf Novae and the Auckland Observatory Programme - W. S. G. Walker. Southern Stars Vol. 24 No. 7 p 126-129 An Outburst of VW Hydri Brian F. Marino and W. S. G. Walker.