Uncalibrated lightcurves of the Marsden group comets 1999 J6 (bottom), 2004 V9 (middle), and 2010 H3 (top). Observations in the C2 orange filter are plotted as orange triangles, C3 clear filter observations are plotted as green circles, and C3 orange filter observations are plotted as orange asterisks. Error bars have not been plotted. The lightcurve is virtually identical on all three apparitions and is much brighter than any other observed Marsden comet (Figures B14-B17 in Knight 2008[PDF]), making their identities as the same comet very likely.
The comet was first observed in C2 on 20100418 at 22:30 UT and was observed until 20100419 13:54 UT. It was then visible in C3 from 20100419 14:18 UT until at least 20100420 16:18 UT. Note that the magnitudes for the 2010 apparition are preliminary as they were determined from SOHO "quick-look" images. The comet appears to be slightly fainter each apparition, but the lightcurves should not be directly compared until the effects of changing viewing geometry have been accounted for and the processed "level 0.5" images are made available by the SOHO team. Because the period is ~5.5 years, the viewing geometries in 1999 and 2010 are virtually identical and approximately opposite the viewing geometry in 2004.
The discontinuity in brightness as the comet moves from the C2 telescope to the C3 telescope is due to the different bandpasses of the C2 orange and C3 clear filters. Sungrazing comets tend to appear ~1 mag brighter in the orange filter than the clear filter, most likely due to sodium emission (Biesecker et al. 2002, Knight et al. 2010).
These findings were published on CBET 2256 and IAUC 9138.
UPDATE 5/2/10: The initial version of this plot had an erroneous zero point for the C3 orange filter images, making them appear 0.761 mag fainter than they should have. These have been corrected, and C3 orange filter points have been added.
Thanks to Rainer Kracht for notifying me of the recovery in 2010 and to Karl Battams for x,y positions of the comet.