To calculate the positions of each of the components of these triple images the fitting with a model consisting on the sum of three bivariate Gaussian distributions was applied (Dick et al. 1993). The density Di,j, measured in pixel i,j, is then:
In this expression B is the background of the frame. As the triple images are
confined to areas of 
mm at most, B can be considered
constant in each of these frames (see Sect. 6 (click here)). Ak is the peak
density of the k-th image, while
xij,yij are the pixel coordinates and xck,yck are the centre
coordinates of the k-th image. 
and tk are
the parameters
of an ellipse with arbitrary orientation and axis size.
These three parameters are related to the ellipse semimajor and semiminor
axis, ak and bk, by the expressions:
and the angle between X and ak axes is
where
The parameter sk is a flattening parameter which takes into account the saturation of the photographic emulsion.
The fitting process was performed by means of the Levenberg-Marquardt method
(Marquardt 1963) for non-linear least squares fitting to work
out the values of the 22 free parameters in the model. The fitting was
applied to small frames of 
mm (
pixels),
or mm (
pixels) for bright stars, around
each set of triple images. One of our main objectives is to reduce the
number of these free parameters in order to achieve better results (see
Sect. 6 (click here)). At the moment this software has not been fully prepared
for its use by other groups, but we intend to make it available to the
community as soon as possible.
