Comet 133P/Elst-Pizarro is the first-known and currently best-characterised member of the main-belt comets , a recently-identified class of objects that exhibit cometary activity but which are dynamically indistinguishable from main-belt asteroids .
We report here on the results of a multi-year monitoring campaign from 2003 to 2008 , and present observations of the return of activity in 2007 .
We find a pattern of activity consistent with the seasonal activity modulation hypothesis proposed by Hsieh et al .
( 2004 , AJ , 127 , 2997 ) .
Additionally , recomputation of phase function parameters using data in which 133P was inactive yields new IAU parameters of H _ { R } = 15.49 \pm 0.05 mag and G _ { R } = 0.04 \pm 0.05 , and linear parameters of m _ { R } ( 1 , 1 , 0 ) = 15.80 \pm 0.05 mag and \beta = 0.041 \pm 0.005 mag deg ^ { -1 } .
Comparison between predicted magnitudes using these new parameters and the comet ’ s actual brightnesses during its 2002 and 2007 active periods reveals the presence of unresolved coma during both episodes , on the order of \sim 0.20 of the nucleus cross-section in 2002 and \sim 0.25 in 2007 .
Multifilter observations during 133P ’ s 2007 active outburst yield mean nucleus colours of B - V = 0.65 \pm 0.03 , V - R = 0.36 \pm 0.01 , and R - I = 0.32 \pm 0.01 , with no indication of significant rotational variation , and similar colours for the trail .
Finally , while 133P ’ s trail appears shorter and weaker in 2007 than in 2002 , other measures of activity strength such as dust velocity and coma contamination of nucleus photometry are found to remain approximately constant .
We attribute changes in trail strength to the timing of observations and projection effects , thus finding no evidence of any substantial decrease in activity strength between 2002 and 2007 .