GJ 436 (also known as Tycho 1984 2613 1) is a 10th magnitude star in Leo approximately 33 light years away. It is a red dwarf whose luminosity is about 0.5 percent of the Sun’s. A Neptune-sized planet has been detected and another planet is suspected based upon variations in the observed transits of the first.
Based upon previous measurements a transit was predicted for Friday night, February 29, 2008 CST (March 1, 2008 UT) so I set up to observe the star using the 80-cm telescope of the ASEM observatory. Over a two hour span centered on the predicted transit I took 300 images with the I filter and reduced the data in MaximDL using reference stars provided by the AAVSO. The standard deviation of the reference stars over this span was 0.006 magnitude - not bad for 10 second exposures! I subtracted the observed measurements from the average of all of them so that up is brighter and down is dimmer in the plot below.
The transit is subtle, but real. By visual inspection, I selected the regions shown by the thick horizontal bars and calculated the average over that span. The change in brightness is 0.009 magnitudes (0.007 predicted). While the standard deviation of the individual data points is some 0.01 magnitudes, the standard deviation for these averages is some 10-12 times better than that, so on the order of 0.001 magnitudes. While the depth of the transit is fairly reliable, the onset and duration are more suspect given that the values were estimated visually. Nevertheless, the estimated start of the transit was 20080301 3h28m47s (some 27 minutes later than predicted) and the estimated duration was some 46 minutes. It is that lag in the start of the transit that will yield information about a second planet - if it exists.