  We employ a large sample of 20171 optically-selected groups and clusters at
0.15 < z < 0.4 in the SDSS to investigate how the stacked stellar mass fraction
varies across a wide range of total mass, $M_{500}$. Our study improves upon
previous observational studies in a number of important ways, including having
a much larger sample size, an explicit inclusion of the intracluster light
(ICL) component, and a thorough examination of the accuracy of our total mass
estimates via comparisons to simulations and weak lensing observations. We find
that the stellar mass fraction depends only weakly on total mass and that the
contribution of ICL to the total stellar mass fraction is significant
(typically 20-40 per cent). Both of these findings are in excellent accordance
with the predictions of cosmological simulations. Under the assumption of a
Chabrier (Salpeter) IMF, the derived star formation efficiency
($f_{star}$/$f_{b}$, where $f_b=\Omega_b/\Omega_m$) is relatively low at 8 per
cent (14 per cent) and is consistent with the global star formation efficiency
of semi-analytic models that reproduce the galaxy stellar mass function. When
our measured stellar mass fractions are combined with the observed relation
between hot gas mass fraction and total mass from X-ray observations, our
results imply that galaxy groups have significantly lower baryon fractions than
massive clusters. Ejection of gas due to energetic AGN feedback (most likely at
high redshift) provides a plausible mechanism for explaining the trends we
observe.
