On the Search for Transits of the Planets Orbiting Gliese 876

We report the results of a globally coordinated photometric campaign tosearch for transits by the P ~ 30 d and P ~ 60 d outer planets of the 3-planetsystem orbiting the nearby M-dwarf Gl 876. These two planets experience strongmutual perturbations, which necessitate use of a dynamical (four-body) model tocompute transit ephemerides for the system. Our photometric data have beencollected from published archival sources, as well as from our photometriccampaigns that were targeted to specific transit predictions. Our analysisindicates that transits by planet "c" (P ~ 30 d) do not currently occur, inconcordance with the best-fit i = 50 degree co-planar configuration obtained bydynamical fits to the most recent radial velocity data for the system. Transitsby planet "b" (P ~ 60 d) are not entirely ruled out by our observations, butour data indicate that it is very unlikely that they occur. Our experience withthe Gl 876 system suggests that a distributed ground-based network of smalltelescopes can be used to search for transits of very low mass M-stars byterrestrial-sized planets.Comment: currently 17pp w/Figs, 10 figures; to appear in Astrophysical Journal article December 2006 v653n