The starting point for this work was the dataset obtained during the optical longtermmonitoring program of comet C/1995 O1 Hale-Bopp [Rauer et al., 1997, 2002]. Observations started in April 1996 and ended nearly 5 years later in January 2001 covering a heliocentric distance range from 4.6-2.8 AU pre- perihelion and from 2.9-12.8 AU post-perihelion. It was the longest campaign ever observing a single comet in the optical wavelengths range so far. This huge dataset had to be reduced and

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... During a first analysis using the Haser model a number of scientifically interesting questions arose, which had to be analyzed further in more detailed studies. One of these was especially interesting: the formation of C2 and C3 in a cometary coma. C2 has been the first constituent of the coma to be found by spectroscopy observations. This was done already in 1864 by Giovanni Donati [1864] in comet Tempel (1864 II) and independently in 1867 by Sir William Huggins [1867] in comet Winnecke (1867 II). Both identified the emissions by comparing the cometary spectra with flame spectra. They also observed a group of lines near 4050 A, but it took more than eighty years until Douglas [1951] identified these lines as C3 emissions. For more than a century C2 remained without an observed parent molecule. A number of parent molecules had been proposed, namely acetylene C2H2 and ethane C2H6 for C2 [Jackson, 1976] and propyne or its isomeric form allene C3H4 for C3 [Stief et al., 1972]. Finally in 1996 emissions of C2H2 and C2H6 have been detected in comets Hyakutake and Hale- Bopp [Tokunaga et al., 1996; Brooke et al., 1996; Mumma et al., 1996]. C3 is up to the present without an observed parent molecule. The main aim of this study is to analyse the formation of C2 and C3 in a cometary coma at large heliocentric distances. As Crovisier and Encrenaz [2000] pointed out in their book on comets: C2 has been an orphan for nearly a century. To stay in this picture, this work will help to link the 'orphan' with its heritage by providing a f [...]