Many of the stars occulted by the Moon are in fact double. Occasionally the stars making up the pair are wide enough apart to be seen as separate objects, and the occultations of each star can be easily timed as separate events. In other cases the two stars are so close that they appear as a single object. Even then the presence of two stars may be detected at occultation as a stepped event - the image of the stars disappear in stages, separated usually by a fraction of a second.

An example of both is presented by star σ (sigma) Scorpii, which has 3 components. Sigma itself is a bright 3rd magnitude star. It has long been known to have a nearby neighbouring star, more than 5 magnitudes fainter and some 20 arc-seconds away. The bright, primary star was discovered to be, itself, a close double in 1972, initially by an occultation. The two components are less than half an arc-second apart and because of their brightness, magnitudes 3.07 and 5.16, are very difficult to separate visually. Occultations may easily show them.

Below are frames from a video of one such event, in 2005. The 5th magnitude secondary star is easily seen as a smaller image in the third frame, after the disappearance of the bright primary star which hides it in the first two frames. This is the same star that was grazed by the Moon at the end of July 2009 with a path crossing NZ from near Jackson's Bay in Fjordland to the southern Wairarapa. It was observed by a number of amateur astronomers equipped with videos in both Islands in poor conditions.

A video record of such an event can be analysed, using a program called Limovie, for the change in light level for the star and a light curve obtained. If the star is double a clear step may be visible. The graph below shows the drop in light for the occultation of the close pair of star forming sigma Scorpii.

Few double stars show stepped events as clearly as this. But many can be detected during occultations using video techniques, showing pairs which are not detectable as such by eye. Discoveries of previously unknown double are still being made by amateurs with modest telescopes using such techniques. Accurate measurements of the times of each step, made by two or more well spaced observers can give very accurate determinations of the separation and position angle, that is the angle they are to each other. These are two quantities which astronomically define the pair. Also measurements of the sizes of the steps can be used to determine the magnitudes of each of the stars.

All this can be done with quite simple equipment. The results are publishable, and are used by professional astronomers. There is a need for the observations. The list for the month show stars which may be observable as double. If you attempt video observations of any of these, please send results to Brian Loader, even if you see no double event.

If you use the OCCULT program to prepare occultation observations, there is a facility to send double star reports automatically, together with a light curve if you have prepared one.