  We compute masses and densities for ten periodic comets with known sizes:
1P/Halley, 2P/Encke, 6P/d'Arrest, 9P/Tempel 1, 10P/Tempel 2, 19P/Borrelly,
22P/Kopff, 46P/Wirtanen, 67P/Churyumov-Gerasimenko and 81P/Wild 2. The method
follows the one developed by Rickman and colleagues (Rickman 1986, 1989;
Rickman et al. 1987), which is based on the gas production curve and on the
change in the orbital period due to the non-gravitational force. The gas
production curve is inferred from the visual lightcurve. We found that the
computed masses cover more than three orders of magnitude: ~(0.3 - 400)*10^12
kg. The computed densities are in all cases very low (<= 0.8 g cm^-3), with an
average value of 0.4 g cm^-3, in agreement with previous results and models of
the cometary nucleus depicting it as a very porous object. The computed comet
densities turn out to be the lowest among the different populations of solar
system minor bodies, in particular as compared to those of near-Earth asteroids
(NEAs). We conclude that the model applied in this work, in spite of its
simplicity (as compared to more sophisticated thermophysical models applied to
very few comets), is useful for a statistical approach to the mean density of
the cometary nuclei. However, we cannot assess from this simple model if there
is a real dispersion among the bulk densities of comets that could tell us
about differences in physical structure (porosity) and/or chemical composition.
