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9 Conclusions

This catalog represents the first comprehensive compilation of nuclear magnitudes of JF comets from different sources. Previous work on this subject was scanty and referred exclusively to the material collected by a few prolific observers, as for instance Roemer or Jewitt. As an example, Shoemaker & Wolfe (1982) derived the luminosity function of JF comets based only on Roemer's data. A comparison with their results is presented elsewhere (Fernández et al. 1999).

  \begin{figure}
{\resizebox{8cm}{!}{\includegraphics{ds1835f10.ps}} }
\end{figure} Figure 8: The relative change in q as a function of the minimum q during the observed period. The different symbols correspond to: Low-active (L) comets whose changes in q took place in the last five revolutions (large crosses), or between 6-20 revolutions ago or without important changes ( $\delta q \sim 0$) (small crosses); Intermediate-active (I) comets (similarly, large and small triangles); High-active (H) comets (large and small squares); and Very high-active (V) comets (large and small hexagons)

The increase in both quantity and quality of the nuclear magnitude data during the last fifteen years is spectacular, thanks mainly to the introduction of CCD cameras and the implementation of dedicated programs with medium-sized and large telescopes. Almost 2/3 of the data that we present was taken during the 1990's.

We plotted the compiled absolute nuclear magnitudes of 138 JF comets as a function of the heliocentric distances at which they were determined. From these plots we were able to provide our "best estimates'' for the absolute nuclear magnitudes of 105 JF comets. There are in addition other 27 JF comets - most of them lost - without estimated nuclear magnitudes. About 60% of the JF comets of our sample show very high or high activity at large heliocentric distances (r > 3 AU). This activity is correlated with downward jumps in the perihelion distance.

We find good consistency between Scotti's data (which partly relies on a method of coma substraction) and the rest of the CCD data. Roemer's nuclear magnitudes are on average about two magnitudes brighter than the ones obtained from CCD observations (including Scotti's) and our best estimates. Our adopted values for the nuclear magnitudes of those JF comets observed by the HST show good agreement with the corresponding estimates by the HST observers.

Our aim was to put together different sources of data on nuclear magnitudes of JF comets analysing their consistency and reliability. It is obvious that this effort is a first step, and great improvements in the statistics should be expected in the near future, as more data from HST, space missions, and ground-based observations with the new-generation very large telescopes will be obtained.

The catalog can be accessed online at: http://www.fisica.edu.uy/~gonzalo/catalog/. The plots for each comet can be accessed separately. We plan to include the raw and reduced data as plain text. Further updates of this catalog will be presented in the Web page.

Acknowledgements
The authors want to thank James Scotti and Lars Kamél for fruitful discussions on comet photometry, and Rudolph Dvorak for his hospitality during our stay at the Vienna Observatory during which part of this paper was carried out. Financial support from the Program for the Development of Basic Science (PEDECIBA) and the National Research Council of Uruguay (CONICYT) is gratefully acknowledged.


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